#include <bits/stdc++.h>
using namespace std;
using ll=long long;
#define f first
#define s second
#define pii pair<int,int>
#define all(a) a.begin(),a.end()
#define sz(a) (int)a.size()
const int N=405;
int n;
string s;
int cntR,cntG,cntY;
int pre[3][N]; //số phần tử X từ 1 ->i;
int id[3][N]; //vi tri cua phần tử x thứ i
int dp[3][N][N][N];
const int INF=INT_MAX-50;
void presolve(){
for(int i=1;i<=n;++i){
for(int j=0;j<3;++j) pre[j][i]+=pre[j][i-1];
if(s[i]=='R'){
++cntR;
++pre[0][i];
id[0][cntR]=i;
}
if(s[i]=='G'){
++cntG;
++pre[1][i];
id[1][cntG]=i;
}
if(s[i]=='Y'){
++cntY;
++pre[2][i];
id[2][cntY]=i;
}
}
for (int c = 0; c < 3; ++c)
for (int r = 0; r <= cntR; ++r)
for (int g = 0; g <= cntG; ++g)
for (int y = 0; y <= cntY; ++y)
dp[c][r][g][y] = INF;
for (int c = 0; c < 3; ++c)
dp[c][0][0][0] = 0;
}
void solve(){
cin>>n>>s;
s='%'+s;
presolve();
for(int i=0;i<=cntR;++i){
for(int j=0;j<=cntG;++j){
for(int k=0;k<=cntY;++k){
for(int cur=0;cur<3;++cur){
if (dp[cur][i][j][k] == INF) continue;
for(int nx=0;nx<3;++nx){
if(cur==nx) continue;
if(i==cntR && nx==0)continue;
if(j==cntG && nx==1)continue;
if(k==cntY && nx==2)continue;
int nr=i,ng=j,ny=k;
int c=0;
if(nx==0){
++nr;
c+=max(0,ng-pre[1][id[0][nr]]);
c+=max(0,ny-pre[2][id[0][nr]]);
}
if(nx==1){
++ng;
c+=max(0,nr-pre[0][id[1][ng]]);
c+=max(0,ny-pre[2][id[1][ng]]);
}
if(nx==2){
++ny;
c+=max(0,ng-pre[1][id[2][ny]]);
c+=max(0,nr-pre[0][id[2][ny]]);
}
dp[nx][nr][ng][ny]=min(dp[nx][nr][ng][ny],dp[cur][i][j][k]+c);
}
}
}
}
}
int res = INF;
for (int i = 0; i <= 2; ++i)
res = min(res, dp[i][cntR][cntG][cntY]);
cout << (res == INF ? -1 : res);
}
signed main(){
ios_base::sync_with_stdio(0);cin.tie(0);
solve();
}
