#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
int key(char c){
if (c == 'R') return 0;
if (c == 'G') return 1;
if (c == 'Y') return 2;
}
const int INF = 1e9 + 9;
int main(){
cin.tie(0)->sync_with_stdio(0);
int n; cin >> n;
string s; cin >> s;
vector<int> pos[3];
for (int i = 0; i < n; i++){
pos[key(s[i])].push_back(i);
}
int R = pos[0].size(), G = pos[1].size(), Y = pos[2].size();
auto val = [&](vector<int> &a, int x, int k){
int ret = lower_bound(a.begin(), a.end(), x) - a.begin();
return max(0, ret - k);
};
vector<vector<vector<int>>> dp(n + 1, vector<vector<int>>(n + 1, vector<int>(3, INF)));
dp[0][0][0] = dp[0][0][1] = dp[0][0][2] = 0;
for (int t = 1; t <= n; t++){
vector<vector<vector<int>>> tmp(n + 1, vector<vector<int>>(n + 1, vector<int>(3, INF)));
for (int i = 1; i <= t && i <= R; i++){
for (int j = 0; i + j <= t && j <= G; j++){
int k = t - i - j;
if (k <= Y)
tmp[i][j][0] = min(tmp[i][j][0], min(dp[i - 1][j][1], dp[i - 1][j][2]) + val(pos[1], pos[0][i - 1], j) + val(pos[2], pos[0][i - 1], k));
}
}
for (int i = 0; i <= t && i <= R; i++){
for (int j = 1; i + j <= t && j <= G; j++){
int k = t - i - j;
if (k <= Y)
tmp[i][j][1] = min(tmp[i][j][1], min(dp[i][j - 1][0], dp[i][j - 1][2]) + val(pos[0], pos[1][j - 1], i) + val(pos[2], pos[1][j - 1], k));
}
}
for (int i = 0; i <= t && i <= R; i++){
for (int j = 0; i + j <= t && j <= G; j++){
int k = t - i - j;
if (0 < k && k <= Y)
tmp[i][j][2] = min(tmp[i][j][2], min(dp[i][j][0], dp[i][j][1]) + val(pos[0], pos[2][k - 1], i) + val(pos[1], pos[2][k - 1], j));
}
}
swap(dp, tmp);
}
int ans = min({dp[R][G][0], dp[R][G][1], dp[R][G][2]});
cout << (ans == INF? -1: ans) << '\n';
return 0;
}
