#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int MX = 405;
int N;
string s;
int dp[MX][MX][MX][4];
int cnt[MX][3]; //R = 0, G = 1, Y = 2
vector<int> R, G, Y;
int f(int r, int g, int y, int l) {
if(r + g + y == N) return 0;
if(dp[r][g][y][l] != -1) return dp[r][g][y][l];
int res = 1e9;
if(r < R.size() && l != 0) {
int cost = max(0, cnt[R[r]][1] - g) + max(0, cnt[R[r]][2] - y);
res = min(res, f(r + 1, g, y, 0) + cost);
}
if(g < G.size() && l != 1) {
int cost = max(0, cnt[G[g]][0] - r) + max(0, cnt[G[g]][2] - y);
res = min(res, f(r, g + 1, y, 1) + cost);
}
if(y < Y.size() && l != 2) {
int cost = max(0, cnt[Y[y]][0] - r) + max(0, cnt[Y[y]][1] - g);
res = min(res, f(r, g, y + 1, 2) + cost);
}
// cout << r << " " << g << " " << y << " = " << res << '\n';
return dp[r][g][y][l] = res;
}
int main() {
cin.tie(0); ios_base::sync_with_stdio(0);
memset(dp, -1, sizeof dp);
cin >> N >> s;
for(int i = 0; i < N; i++) {
if(s[i] == 'R') {
R.push_back(i);
cnt[i][0]++;
}
if(s[i] == 'G') {
G.push_back(i);
cnt[i][1]++;
}
if(s[i] == 'Y') {
Y.push_back(i);
cnt[i][2]++;
}
if(i > 0) {
cnt[i][0] += cnt[i - 1][0];
cnt[i][1] += cnt[i - 1][1];
cnt[i][2] += cnt[i - 1][2];
}
}
int k = f(0, 0, 0, 3);
if(k == 1e9) k = -1;
cout << k << '\n';
}
