#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int MX = 407;
int N;
ll dp[MX][MX][MX][3];
vector<int> R, G, Y;
ll f(int r, int g, int y, int c) {
ll pos = r + g + y, res = 1e18;
if(dp[r][g][y][c] != -1) return dp[r][g][y][c];
if(pos == N) return dp[r][g][y][c] = 0;
if(c != 0 && r + 1 < R.size()) {
res = min(res, f(r + 1, g, y, 0) + max(0LL, R[r + 1] - pos - 1));
}
if(c != 1 && g + 1 < G.size()) {
res = min(res, f(r, g + 1, y, 1) + max(0LL, G[g + 1] - pos - 1));
}
if(c != 2 && y + 1 < Y.size()) {
res = min(res, f(r, g, y + 1, 2) + max(0LL, Y[y + 1] - pos - 1));
}
return dp[r][g][y][c] = res;
}
int main() {
cin.tie(0); ios_base::sync_with_stdio(0);
memset(dp, -1, sizeof dp);
cin >> N;
R = {0}, G = {0}, Y = {0};
for(int i = 1; i <= N; i++) {
char c;
cin >> c;
if(c == 'R') R.push_back(i);
if(c == 'G') G.push_back(i);
if(c == 'Y') Y.push_back(i);
}
ll k = min({f(0, 0, 0, 0), f(0, 0, 0, 1), f(0, 0, 0, 2)});
if(k == 1e18) k = -1;
cout << k << '\n';
}
