#include <bits/stdc++.h>
using namespace std;
int n;
string s;
int r, g, b;
const int MXN = 405;
int dp[MXN][MXN][MXN][4];
vector<int> rs, gs, bs;
signed main() {
cin >> n >> s;
int rc = 0, bc = 0, gc = 0;
for (int i=0; i<n; i++) {
if (s[i] == 'R') {
rs.push_back(i+1);
r++;
rc++;
}
if (s[i] == 'G') {
gs.push_back(i+1);
g++;
gc++;
}
if (s[i] == 'Y') {
bs.push_back(i+1);
b++;
bc++;
}
}
s = ' ' + s;
for (int ind=1; ind<=n; ind++) {
for (int r=0; r<=rc; r++) {
for (int g=0; g<=gc; g++) {
int b = ind - r - g;
if (b > bc || b < 0) {
for (int whi=0; whi<=3; whi++) {
dp[ind][r][g][whi] = INT_MAX / 2;
}
}
else {
for (int whi=0; whi<=3; whi++) {
///
int bt = INT_MAX / 2;
if (r > 0 && whi != 1) {
bt = min(bt, (dp[ind-1][r-1][g][1] + abs(rs[r-1] - ind)));
}
if (g > 0 && whi != 2) {
bt = min(bt, (dp[ind-1][r][g-1][2] + abs(gs[g-1] - ind)));
}
if (b > 0 && whi != 3) {
bt = min(bt, (dp[ind-1][r][g][3] + abs(bs[b-1] - ind)));
}
dp[ind][r][g][whi] = bt;
///
}
}
}
}
}
int re = dp[n][r][g][0];
if (re > n * n * 2) {
cout << -1 << endl;
}
else {
cout << re / 2 << endl;
}
}
