#include <bits/stdc++.h>
#define ll long long
using namespace std;
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
ll n;
cin >> n;
string s;
cin >> s;
vector<ll> pos[3];
ll cnt[3][407];
for (int i=0; i<3; i++) cnt[i][0] = 0;
for (int i=0; i<n; i++) {
ll idx = -1;
if (s[i]=='R') idx = 0;
else if (s[i]=='G') idx = 1;
else if (s[i]=='Y') idx = 2;
pos[idx].push_back(i);
for (int j=0; j<3; j++) {
cnt[j][i+1] = cnt[j][i]+(j==idx);
}
}
ll n1 = pos[0].size();
ll n2 = pos[1].size();
ll n3 = pos[2].size();
vector<vector<vector<vector<ll>>>> dp(n1+1, vector<vector<vector<ll>>>(n2+1, vector<vector<ll>>(n3+1, vector<ll>(4, 1e9))));
dp[0][0][0][3] = 0;
for (int r=0; r<=n1; r++) {
for (int g=0; g<=n2; g++) {
for (int y=0; y<=n3; y++) {
for (int i=0; i<4; i++) {
ll curr = dp[r][g][y][i];
if (curr==1e9) continue;
if (r<n1 && i!=0) {
ll p = pos[0][r];
ll c = max(0LL, cnt[1][p]-g)+max(0LL, cnt[2][p]-y);
dp[r+1][g][y][0] = min(dp[r+1][g][y][0], curr+c);
}
if (g<n2 && i!=1) {
ll p = pos[1][g];
ll c = max(0LL, cnt[0][p]-r)+max(0LL, cnt[2][p]-y);
dp[r][g+1][y][1] = min(dp[r][g+1][y][1], curr+c);
}
if (y<n3 && i!=2) {
ll p = pos[2][y];
ll c = max(0LL, cnt[0][p]-r)+max(0LL, cnt[1][p]-g);
dp[r][g][y+1][2] = min(dp[r][g][y+1][2], curr+c);
}
}
}
}
}
ll res = 1e9;
for (int i=0; i<3; i++) res = min(res, dp[n1][n2][n3][i]);
if (res==1e9) cout << -1;
else cout << res;
}
