#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef double lf;
typedef pair<ll,ll> ii;
#define REP(i,n) for(int i=0;i<n;i++)
#define REP1(i,n) for(ll i=1;i<=n;i++)
#define RST(i,n) memset(i,n,sizeof i)
#define SZ(a) (int)a.size()
#define ALL(a) a.begin(),a.end()
#define X first
#define Y second
#define mkp make_pair
#define pb push_back
#define eb emplace_back
#define pob pop_back
#ifdef cold66
#define debug(...) do{\
fprintf(stderr,"%d (%s) = ",__LINE__,#__VA_ARGS__);\
_do(__VA_ARGS__);\
}while(0)
template<typename T>void _do(T &&_x){cerr<<_x<<endl;}
template<typename T,typename ...S> void _do(T &&_x,S &&..._t){cerr<<_x<<", ";_do(_t...);}
template<typename _a,typename _b> ostream& operator << (ostream &_s,const pair<_a,_b> &_p){return _s<<"("<<_p.X<<","<<_p.Y<<")";}
template<typename It> ostream& _OUTC(ostream &_s,It _ita,It _itb)
{
_s<<"{";
for(It _it=_ita;_it!=_itb;_it++)
{
_s<<(_it==_ita?"":",")<<*_it;
}
_s<<"}";
return _s;
}
template<typename _a> ostream &operator << (ostream &_s,vector<_a> &_c){return _OUTC(_s,ALL(_c));}
template<typename _a> ostream &operator << (ostream &_s,set<_a> &_c){return _OUTC(_s,ALL(_c));}
template<typename _a,typename _b> ostream &operator << (ostream &_s,map<_a,_b> &_c){return _OUTC(_s,ALL(_c));}
template<typename _t> void pary(_t _a,_t _b){_OUTC(cerr,_a,_b);cerr<<endl;}
#define IOS()
#else
#define debug(...)
#define pary(...)
#define endl '\n'
#define IOS() ios_base::sync_with_stdio(0);cin.tie(0);
#endif // cold66
//}
template<class T> inline bool chkmax(T &a, const T &b) { return b > a ? a = b, true : false; }
template<class T> inline bool chkmin(T &a, const T &b) { return b < a ? a = b, true : false; }
template<class T> using MaxHeap = priority_queue<T>;
template<class T> using MinHeap = priority_queue<T, vector<T>, greater<T>>;
const ll MAXn=4e2+5,MAXlg=__lg(MAXn)+2;
const ll MOD=1000000007;
const ll INF=0x3f3f3f3f;
int R,G,Y;
int rg[MAXn],ry[MAXn];
int gr[MAXn],gy[MAXn];
int yr[MAXn],yg[MAXn];
int dp[MAXn][MAXn][MAXn][3];
int main(){
IOS();
int n;
cin >> n;
string s;
cin >> s;
REP (i,n) {
if (s[i] == 'R') {
rg[R] = G;
ry[R++] = Y;
}
else if (s[i] == 'G') {
gr[G] = R;
gy[G++] = Y;
}
else {
yr[Y] = R;
yg[Y++] = G;
}
}
debug(R,G,Y);
RST(dp,INF);
REP (i,3) dp[0][0][0][i] = 0;
REP (i,R+1) REP (j,G+1) REP (k,Y+1) REP (l,3) {
int ret = dp[i][j][k][l];
if (i < R && l != 0) {
dp[i+1][j][k][0] = min(dp[i+1][j][k][0],ret + max(rg[i]-j,0) + max(ry[i]-k,0));
}
if (j < G && l != 1) {
dp[i][j+1][k][1] = min(dp[i][j+1][k][1],ret + max(gr[j]-i,0) + max(gy[j]-k,0));
}
if (k < Y && l != 2) {
dp[i][j][k+1][2] = min(dp[i][j][k+1][2],ret + max(yr[k]-i,0) + max(yg[k]-j,0));
}
}
int ans = INF;
REP (i,3) ans = min(ans,dp[R][G][Y][i]);
if (ans == INF) cout << -1 << endl;
else cout << ans << endl;
}
