#include <bits/stdc++.h>
using namespace std;
#define pb push_back
void chmin(int& x,int y){x=min(x,y);}
const int maxn = 400;
int pre[maxn][3];
int dp[maxn+5][maxn+5][maxn+5][3];
int main(){
ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);
int n;
cin>>n;
string s;
cin>>s;
vector<int>a(n),cnt(3,0);
vector<int>pos[3];
for(int i=0;i<n;i++){
if(s[i]=='R') a[i]=0;
else if(s[i]=='G'){
a[i]=1;
}
else if (s[i] == 'Y'){
a[i]=2;
}
pos[a[i]].pb(i);
cnt[a[i]]++;
}
for(int i = 0 ; i <=n;i++){
for(int j = 0 ; j <= n ; j++){
for(int k = 0 ; k <=n ; k++){
for(int c=0;c<3;c++){
dp[i][j][k][c]=1e9;
}
}
}
}
pre[0][0]=pre[0][1]=pre[0][2]=0;
dp[0][0][0][0]=dp[0][0][0][1]=dp[0][0][0][2]=0;
pre[0][a[0]]++;
for(int i = 1 ; i < n ; i++){
for(int c=0;c<3;c++){
pre[i][c] =pre[i-1][c];
if (a[i] == c){
pre[i][c] += 1;
}
}
}
for(int i=0 ; i <= cnt[0] ; i ++){
for(int j = 0 ; j <= cnt[1] ; j ++){
for(int k=0;k<=cnt[2];k++){
for(int c=0;c<3;c++){
if(i<cnt[0] and c!=0){
int p=pos[0][i];
dp[i+1][j][k][0] = min(dp[i+1][j][k][0],dp[i][j][k][c]+max(pre[p][1]-j,0)+max(pre[p][2]-k,0));
}
if(k<cnt[2] and c!=2){
int p=pos[2][k];
dp[i][j][k+1][2] = min(dp[i][j][k+1][2],dp[i][j][k][c]+max(pre[p][0]-i,0)+max(pre[p][1]-j,0));
}
if(j<cnt[1] and c!=1){
int p=pos[1][j];
dp[i][j+1][k][1] = min(dp[i][j+1][k][1],dp[i][j][k][c]+max(pre[p][2]-k,0)+max(pre[p][0]-i,0));
}
}
}
}
}
long long ans = 1e9;
for (int i = 0 ; i < 3 ; i ++){
ans = min (ans , dp[cnt[0]][cnt[1]][cnt[2]][i]*1ll);
}
if(ans == 1e9){
ans=-1;
}
cout<<ans;
}
