#include <bits/stdc++.h>
using namespace std;
#define int long long
#define FOR(i,s,e) for(int i = s; i <= (int)e; ++i)
#define DEC(i,s,e) for(int i = s; i >= (int)e; --i)
#define IAMSPEED ios_base::sync_with_stdio(false); cin.tie(0);
#ifdef LOCAL
#define db(x) cerr << #x << "=" << x << "\n"
#define db2(x, y) cerr << #x << "=" << x << " , " << #y << "=" << y << "\n"
#define db3(a,b,c) cerr<<#a<<"="<<a<<","<<#b<<"="<<b<<","<<#c<<"="<<c<<"\n"
#define dbv(v) cerr << #v << ":"; for (auto ite : v) cerr << ite << ' '; cerr <<"\n"
#define dbvp(v) cerr << #v << ":"; for (auto ite : v) cerr << "{" << ite.f << ',' << ite.s << "} "; cerr << "\n"
#define dba(a,ss,ee) cerr << #a << ":"; FOR(ite,ss,ee) cerr << a[ite] << ' '; cerr << "\n"
#define reach cerr << "LINE: " << __LINE__ << "\n";
#else
#define reach
#define db(x)
#define db2(x,y)
#define db3(a,b,c)
#define dbv(v)
#define dbvp(v)
#define dba(a,ss,ee)
#endif
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
#define pb push_back
#define eb emplace_back
#define all(x) (x).begin(), (x).end()
#define f first
#define s second
#define g0(x) get<0>(x)
#define g1(x) get<1>(x)
#define g2(x) get<2>(x)
#define g3(x) get<3>(x)
typedef pair <int, int> pi;
typedef tuple<int,int,int> ti3;
typedef tuple<int,int,int,int> ti4;
int rand(int a, int b) { return a + rng() % (b-a+1); }
const int MOD = 1e9 + 7;
const int inf = (int)1e9 + 500;
const long long oo = (long long)1e18 + 500;
template <typename T> bool chmax(T& a, const T b) { return a<b ? a = b, 1 : 0; }
template <typename T> bool chmin(T& a, const T b) { return a>b ? a = b, 1 : 0; }
const int MAXN = 402;
int dp[MAXN][MAXN][MAXN][3];
map<pi, int> C[3][MAXN];//number of positions i have moved forward
char A[MAXN];
vector<int>V[3];
int where[3][MAXN];
int32_t main()
{
IAMSPEED
int n; cin >> n;
FOR(i,1,n) {
char c; cin >> c;
if(c=='R')A[i]=0;
else if(c=='G')A[i]=1;
else A[i]=2;
V[A[i]].pb(i);
}
FOR(zz,0,2) {
FOR(i,1,V[zz].size()) {
int pos=V[zz][i-1];
set<int> s;
for(auto x:{0,1,2})s.insert(x);
s.erase(zz);
int c1=*s.begin(); s.erase(s.begin());
int c2=*s.begin(); s.erase(s.begin());
int cnt=0;
FOR(xx,0,V[c1].size()) {
int id1=0;
if(xx)id1=V[c1][xx-1];
if(id1>pos)++cnt;
int st=cnt;
FOR(yy,0,V[c2].size()) {
int id2=0;
if(yy)id2=V[c2][yy-1];
if(id2>pos)++cnt;
C[zz][i][pi(xx,yy)]=cnt;
}
cnt=st;
}
}
}
FOR(i,0,n)FOR(j,0,n)FOR(k,0,n)FOR(y,0,2)dp[i][j][k][y]=oo;
dp[0][0][0][0]=0;
dp[0][0][0][1]=0;
dp[0][0][0][2]=0;
FOR(i,0,V[0].size()) {
FOR(j,0,V[1].size()){
FOR(k,0,V[2].size()) {
FOR(y,0,2) {
int &cur=dp[i][j][k][y];
if(cur==oo)continue;
if(y!=0){
if(i<V[0].size()){
int curidx=C[0][i+1][pi(j,k)];
chmin(dp[i+1][j][k][0], cur + curidx);
}
}
if(y!=1) {
if(j<V[1].size()) {
int curidx=C[1][j+1][pi(i,k)];
chmin(dp[i][j+1][k][1], cur + curidx);
}
}
if(y!=2){
if(k<V[2].size()) {
int curidx=C[2][k+1][pi(i,j)];
chmin(dp[i][j][k+1][2], cur + curidx);
}
}
}
}
}
}
int r=V[0].size();
int g=V[1].size();
int b=V[2].size();
int ans = min({dp[r][g][b][0], dp[r][g][b][1], dp[r][g][b][2]});
if(ans==oo)cout<<-1;
else cout <<ans;
}
