#include <bits/stdc++.h>
using namespace std;
int N;
vector<int> pos[3];
int pre[3][405];
int dp[3][405][405][405];
int sz[3];
int cst(int p, int t, int x1, int x2, int x3){
int sum = 0;
sum += max(0, pre[0][pos[t][p]] - pre[0][pos[0][x1]]);
sum += max(0, pre[1][pos[t][p]] - pre[1][pos[1][x2]]);
sum += max(0, pre[2][pos[t][p]] - pre[2][pos[2][x3]]);
//cout << sum << "\n";
return sum;
}
int main(){
cin.sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
cin >> N;
pos[0].emplace_back(0), pos[1].emplace_back(0), pos[2].emplace_back(0);
for(int i = 1; i<=N; i++){
char c;
cin >> c;
if(c == 'R'){
sz[0]++;
pos[0].emplace_back(i);
}
else if(c == 'G'){
sz[1]++;
pos[1].emplace_back(i);
}
else{
sz[2]++;
pos[2].emplace_back(i);
}
}
if(max({sz[0], sz[1], sz[2]}) > (N+1)/2){
cout << -1 << "\n";
return 0;
}
for(int c = 0; c<3; c++){
for(int n : pos[c]){
pre[c][n]++;
}
partial_sum(pre[c], pre[c]+N+1, pre[c]);
}
for(int r = 0; r<=sz[0]; r++){
for(int g = 0; g<=sz[1]; g++){
for(int y = 0; y<=sz[2]; y++){
dp[0][r][g][y] = dp[1][r][g][y] = dp[2][r][g][y] = 1000000;
}
}
}
dp[0][0][0][0] = dp[1][0][0][0] = dp[2][0][0][0] = 0;
for(int r = 0; r<=sz[0]; r++){
for(int g = 0; g<=sz[1]; g++){
for(int y = 0; y<=sz[2]; y++){
if(r){
dp[0][r][g][y] = min(dp[1][r-1][g][y], dp[2][r-1][g][y]) + cst(r, 0, r, g, y);
}
if(g){
dp[1][r][g][y] = min(dp[0][r][g-1][y], dp[2][r][g-1][y]) + cst(g, 1, r, g, y);
}
if(y){
dp[2][r][g][y] = min(dp[0][r][g][y-1], dp[1][r][g][y-1]) + cst(y, 2, r, g, y);
}
//cout << dp[0][r][g][y] << " " << dp[1][r][g][y] << " " << dp[2][r][g][y] << "\n";
}
}
}
cout << min({dp[0][sz[0]][sz[1]][sz[2]], dp[1][sz[0]][sz[1]][sz[2]], dp[2][sz[0]][sz[1]][sz[2]]});
}
