#include <bits/stdc++.h>
#define INF 100000000
using namespace std;
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
int n;
cin>>n;
const int N = n+1;
int dp[N][N][N][3];
int g[3][N];
int cc[] = {0, 0, 0};
int cmp[N][3];
if(n==1){
cout<<0<<"\n";
return 0;
}
char c;
for(int i = 1;i<=n;i++){
cmp[i][0] = cc[0];
cmp[i][1] = cc[1];
cmp[i][2] = cc[2];
cin>>c;
if(c=='R') g[0][cc[0]++] = i;
else if(c=='G') g[1][cc[1]++] = i;
else g[2][cc[2]++] = i;
}
memset(dp , INF , sizeof(dp)) ;
for(int i = n-1;i>=1;i--){
for(int aused = n;aused>=0;aused--){
for(int bused = n;bused >= 0 ;bused--){
int cused = i - (aused + bused);
int ause = INF, buse = INF, cuse = INF;
if(aused>cc[0] || bused>cc[1] || cused>cc[2]) continue;
if(cused<0) continue;
int currind;
//calculate use[0]
if(cc[0] > aused) {
currind = g[0][aused] + max((int) 0, bused - cmp[g[0][aused]][1]) +
max((int) 0, cused - cmp[g[0][aused]][2]);
ause = (currind - (i + 1));
if(i<n-1) ause += dp[i + 1][aused + 1][bused][0];
}
//calculate buse
if(cc[1] > bused) {
currind = g[1][bused] + max((int) 0, aused - cmp[g[1][bused]][0]) +
max((int) 0, cused - cmp[g[1][bused]][2]);
buse = (currind - (i + 1));
if(i<n-1) buse += dp[i + 1][aused][bused + 1][1];
}
//calculate cuse
if(cc[2] > cused) {
currind = g[2][cused] + max((int) 0, aused - cmp[g[2][cused]][0]) +
max((int) 0, bused - cmp[g[2][cused]][1]);
cuse = (currind - (i + 1));
if(i<n-1) cuse += dp[i + 1][aused][bused][2];
}
dp[i][aused][bused][0] = min(buse, cuse);
dp[i][aused][bused][1] = min(ause, cuse);
dp[i][aused][bused][2] = min(buse, ause);
}
}
}
int ans = INF;
if(cc[0]) {
ans = min(ans, (g[0][0] - 1) + dp[1][1][0][0]);
}
if(cc[1]) {
ans = min(ans, (g[1][0] - 1) + dp[1][0][1][1]);
}
if(cc[2]) {
ans = min(ans, (g[2][0] - 1) + dp[1][0][0][2]);
}
if(ans>=INF) ans = -1;
cout<<ans<<"\n";
}
