#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int MX = 405;
int N;
string s;
int dp[MX][MX][MX];
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] != -1) return dp[r][g][y];
int res = 1e9;
if(r < R.size() && l != 0) {
auto gg = upper_bound(G.begin(), G.end(), R[r]) - G.begin();
auto yy = upper_bound(Y.begin(), Y.end(), R[r]) - Y.begin();
int cost = max(0, (int)gg - g) + max(0, (int)yy - y);
res = min(res, f(r + 1, g, y, 0) + cost);
}
if(g < G.size() && l != 1) {
auto rr = upper_bound(R.begin(), R.end(), G[g]) - R.begin();
auto yy = upper_bound(Y.begin(), Y.end(), G[g]) - Y.begin();
int cost = max(0, (int)rr - r) + max(0, (int)yy - y);
res = min(res, f(r, g + 1, y, 1) + cost);
}
if(y < Y.size() && l != 2) {
auto rr = upper_bound(R.begin(), R.end(), Y[y]) - R.begin();
auto gg = upper_bound(G.begin(), G.end(), Y[y]) - G.begin();
int cost = max(0, (int)rr - r) + max(0, (int)gg - g);
res = min(res, f(r, g, y + 1, 2) + cost);
}
// cout << r << " " << g << " " << y << " = " << res << '\n';
return 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);
}
if(s[i] == 'G') {
G.push_back(i);
}
if(s[i] == 'Y') {
Y.push_back(i);
}
}
int k = f(0, 0, 0, 3);
if(k == 1e9) k = -1;
cout << k << '\n';
}
