#include <bits/stdc++.h>
using namespace std;
#define VA_NUM_ARGS(...) VA_NUM_ARGS_IMPL_((0,__VA_ARGS__, 6,5,4,3,2,1))
#define VA_NUM_ARGS_IMPL_(tuple) VA_NUM_ARGS_IMPL tuple
#define VA_NUM_ARGS_IMPL(_0,_1,_2,_3,_4,_5,_6,N,...) N
#define macro_dispatcher(macro, ...) macro_dispatcher_(macro, VA_NUM_ARGS(__VA_ARGS__))
#define macro_dispatcher_(macro, nargs) macro_dispatcher__(macro, nargs)
#define macro_dispatcher__(macro, nargs) macro_dispatcher___(macro, nargs)
#define macro_dispatcher___(macro, nargs) macro ## nargs
#define Debug1(a) cout<<#a<<"="<<(a)<<"\n"
#define Debug2(a,b) cout<<#a<<"="<<(a)<<", "<<#b<<"="<<(b)<<"\n"
#define Debug3(a,b,c) cout<<#a<<"="<<(a)<<", "<<#b<<"="<<(b)<<", "<<#c<<"="<<(c)<<"\n"
#define Debug4(a,b,c,d) cout<<#a<<"="<<(a)<<", "<<#b<<"="<<(b)<<", "<<#c<<"="<<(c)<<", "<<#d<<"="<<(d)<<"\n"
#define Debug5(a,b,c,d,e) cout<<#a<<"="<<(a)<<", "<<#b<<"="<<(b)<<", "<<#c<<"="<<(c)<<", "<<#d<<"="<<(d)<<", "<<#e<<"="<<(e)<<"\n"
#define Debug6(a,b,c,d,e,f) cout<<#a<<"="<<(a)<<", "<<#b<<"="<<(b)<<", "<<#c<<"="<<(c)<<", "<<#d<<"="<<(d)<<", "<<#e<<"="<<(e)<<", "<<#f<<"="<<(f)<<"\n"
#define Debug(...) macro_dispatcher(Debug, __VA_ARGS__)(__VA_ARGS__)
#define DA(a,s,n) cout<<#a<<"=["; printarray(a,s,n); cout<<"]\n"
#define TT1 template<class T>
#define TT1T2 template<class T1, class T2>
#define TT1T2T3 template<class T1, class T2, class T3>
template<class T, size_t N> ostream& operator << (ostream& os, const array<T, N>& v);
TT1T2 ostream& operator << (ostream& os, const pair<T1, T2>& p){ return os <<"("<<p.first<<", "<< p.second<<")"; }
TT1 ostream& operator << (ostream& os, const vector<T>& v){ bool f=1;os<<"[";for(auto& i : v) { if (!f)os << ", ";os<<i;f=0;}return os << "]"; }
template<class T, size_t N> ostream& operator << (ostream& os, const array<T, N>& v) { bool f=1;os<<"[";for(auto& i : v) { if (!f)os << ", ";os<<i;f=0;}return os << "]"; }
TT1T2 ostream& operator << (ostream& os, const set<T1, T2>&v){ bool f=1;os<<"[";for(auto& i : v) { if (!f)os << ", ";os<<i;f=0;}return os << "]"; }
TT1T2 ostream& operator << (ostream& os, const multiset<T1,T2>&v){bool f=1;os<<"[";for(auto& i : v) { if (!f)os << ", ";os<<i;f=0;}return os << "]"; }
TT1T2T3 ostream& operator << (ostream& os, const map<T1,T2,T3>& v){ bool f = 1; os << "["; for (auto& ii : v) { if (!f)os << ", "; os << "(" << ii.first << " -> " << ii.second << ") "; f = 0; }return os << "]"; }
TT1T2 ostream& operator << (ostream& os, const multimap<T1, T2>& v){ bool f = 1; os << "["; for (auto& ii : v) { if (!f)os << ", "; os << "(" << ii.first << " -> " << ii.second << ") "; f = 0; }return os << "]"; }
TT1T2 ostream& operator << (ostream& os, priority_queue<T1, T2> v) { bool f = 1; os << "["; while (!v.empty()) { auto x = v.top(); v.pop(); if (!f) os << ", "; f = 0; os << x; } return os << "]"; }
TT1T2 void printarray(const T1& a, T2 l, T2 r){ for (T2 i = l; i<=r; i++) cout << a[i] << (i<r?" ":""); }
void cio(){
ios::sync_with_stdio(false);
cin.tie(NULL);
//cout << setprecision(15);
}
#define tt() printf("%.4f sec\n", (double) clock() / CLOCKS_PER_SEC )
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
#define Fi first
#define Se second
#define pb(x) push_back(x)
#define sz(x) (int)x.size()
#define rep(i, n) for(int i=0;i<n;i++)
#define repp(i, n) for(int i=1;i<=n;i++)
#define all(x) x.begin(), x.end()
#define geti1(X) cin >> X
#define geti2(X,Y) cin >> X >> Y
#define geti3(X,Y,Z) cin >> X >> Y >> Z
#define geti4(X,Y,Z,W) cin >> X >> Y >> Z >> W
#define GET_MACRO(_1,_2,_3,_4,NAME,...) NAME
#define geti(...) GET_MACRO(__VA_ARGS__, geti4, geti3, geti2, geti1) (__VA_ARGS__)
#define endl '\n'
typedef tuple<int,int,int> t3;
typedef tuple<int,int,int,int> t4;
void die(){printf("-1\n"); exit(0);}
int xx[8] = {1,-1,0,0,1,1,-1,-1}, yy[8] = {0,0,1,-1,1,-1,1,-1};
const ll mod = 1e9+7;
#define MAXN 505
int N, p[MAXN], C[4];
int pos[4][MAXN], idx[4];
char buf[MAXN];
int D[MAXN][MAXN][MAXN][3];
int X[3][MAXN][MAXN], Y[MAXN];
void upd(int& a, int b){ if( a > b ) a = b; }
inline int get(int k, int r, int x){
return X[k][r][x];
/*
int low = 1, high = r, res = -1;
while( low <= high ){
int mid = (low+high)/2;
if( pos[k][mid] > x ) res = mid, high = mid-1;
else low = mid+1;
}
if( res == -1 ) return 0;
return r - res + 1;
*/
}
int main(){
scanf("%d",&N); scanf("%s",buf+1);
repp(i,N){
if( buf[i] == 'R' ) p[i] = 0, C[0]++;
if( buf[i] == 'G' ) p[i] = 1, C[1]++;
if( buf[i] == 'Y' ) p[i] = 2, C[2]++;
}
repp(i,N){
pos[p[i]][++idx[p[i]]] = i;
}
for(int k=0;k<3;k++){
repp(i,N) Y[i] = 0;
for(int i=1;i<=idx[k];i++){
for(int j=pos[k][i];j>=1;j--) Y[j]++;
for(int j=1;j<=N;j++) X[k][i][j] = Y[j];
}
}
rep(i,C[0]+1)rep(j,C[1]+1)rep(k,C[2]+1)rep(l,3) D[i][j][k][l] = 1e9;
D[0][0][0][0] = D[0][0][0][1] = D[0][0][0][2] = 0;
int ans = 1e9;
for(int a=0;a<=C[0];a++)
for(int b=0;b<=C[1];b++)
for(int c=0;c<=C[2];c++)
for(int k=0;k<3;k++){
int n = a + b + c;
if( n == N ) ans = min(ans, D[a][b][c][k]);
//if( D[a][b][c][k] == 0 ) Debug(a,b,c,k);
if( a+1 <= C[0] && k != 0 ){
int acc = 0;
acc += get(1, min(b, idx[1]), pos[0][a+1]);
acc += get(2, min(c, idx[2]), pos[0][a+1]);
upd(D[a+1][b][c][0], D[a][b][c][k] + acc);
}
if( b+1 <= C[1] && k != 1 ){
int acc = 0;
acc += get(0, min(a, idx[0]), pos[1][b+1]);
acc += get(2, min(c, idx[2]), pos[1][b+1]);
upd(D[a][b+1][c][1], D[a][b][c][k] + acc);
}
if( c+1 <= C[2] && k != 2 ){
int acc = 0;
acc += get(0, min(a, idx[0]), pos[2][c+1]);
acc += get(1, min(b, idx[1]), pos[2][c+1]);
upd(D[a][b][c+1][2], D[a][b][c][k] + acc);
}
}
if( ans >= 1e9 ) printf("-1\n");
else printf("%d\n",ans);
}
