#include "bits/stdc++.h"
using namespace std;
using ll = long long;
int main() {
cin.tie(0)->sync_with_stdio(0);
int n;
string s;
cin >> n >> s;
vector<int> pr(n + 1), pg(n + 1), py(n + 1);
vector<int> vr, vg, vy;
for (int i = 1; i <= n; i++) {
pr[i] = pr[i - 1];
pg[i] = pg[i - 1];
py[i] = py[i - 1];
if (s[i - 1] == 'R') {
++pr[i];
vr.push_back(i);
}
else if (s[i - 1] == 'G') {
++pg[i];
vg.push_back(i);
}
else if (s[i - 1] == 'Y') {
++py[i];
vy.push_back(i);
}
}
int nr = (int)vr.size();
int ng = (int)vg.size();
int ny = (int)vy.size();
vector dp(nr + 1, vector(ng + 1, vector(ny + 1, vector(3, 1000000000))));
dp[0][0][0][0] = dp[0][0][0][1] = dp[0][0][0][2] = 0;
for (int i = 0; i <= nr; i++) {
for (int j = 0; j <= ng; j++) {
for (int k = 0; k <= ny; k++) {
if (i > 0) {
int pos = vr[i - 1];
dp[i][j][k][0] = min(dp[i][j][k][0], dp[i - 1][j][k][1] + abs(pg[pos] - j) + abs(py[pos] - k));
dp[i][j][k][0] = min(dp[i][j][k][0], dp[i - 1][j][k][2] + abs(pg[pos] - j) + abs(py[pos] - k));
}
if (j > 0) {
int pos = vg[j - 1];
dp[i][j][k][1] = min(dp[i][j][k][1], dp[i][j - 1][k][0] + abs(pr[pos] - i) + abs(py[pos] - k));
dp[i][j][k][1] = min(dp[i][j][k][1], dp[i][j - 1][k][2] + abs(pr[pos] - i) + abs(py[pos] - k));
}
if (k > 0) {
int pos = vy[k - 1];
dp[i][j][k][2] = min(dp[i][j][k][2], dp[i][j][k - 1][0] + abs(pr[pos] - i) + abs(pg[pos] - j));
dp[i][j][k][2] = min(dp[i][j][k][2], dp[i][j][k - 1][1] + abs(pr[pos] - i) + abs(pg[pos] - j));
}
// for (int l = 0; l < 3; l++) {
// cout << i << " " << j << " " << k << " " << l << " : " << dp[i][j][k][l] << endl;
// }
}
}
}
int ans = min(dp[nr][ng][ny][0], min(dp[nr][ng][ny][1], dp[nr][ng][ny][2]));
cout << ((ans >= 10000000) ? -1 : ans / 2) << endl;
}
