#include <bits/stdc++.h>
using namespace std;
#define ll long long
ll dp[401][401][401][3], prefix[400][3];
string plants;
int main() {
int n; cin >> n;
cin >> plants;
int rsum = 0, gsum = 0, ysum = 0;
for (char i: plants) {
if (i == 'R') {rsum++;}
if (i == 'G') {gsum++;}
if (i == 'Y') {ysum++;}
}
if (max(max(rsum, gsum), ysum) * 2 > rsum + gsum + ysum + 1) {
cout << -1; return 0;
}
vector<int> rvec, gvec, yvec;
int rcount = 0, gcount = 0, ycount = 0;
for (int i=0;i<n;i++) {
if (plants[i] == 'R') {
rcount++;
rvec.push_back(i);
}
if (plants[i] == 'G') {
gcount++;
gvec.push_back(i);
}
if (plants[i] == 'Y') {
ycount++;
yvec.push_back(i);
}
prefix[i][0] = rcount;
prefix[i][1] = gcount;
prefix[i][2] = ycount;
}
int rpos, gpos, ypos;
string curplants;
for (int i=0;i<rsum+1;i++) {
for (int j=0;j<gsum+1;j++) {
for (int k=0;k<ysum+1;k++) {
if (i == 0 && j == 0 && k == 0) {continue;}
if (i + j + k == 1) {
dp[i][j][k][0] = INT32_MAX; dp[i][j][k][1] = INT32_MAX; dp[i][j][k][2] = INT32_MAX;
if (i == 1) {
dp[i][j][k][0] = 0;
} else if (j == 1) {
dp[i][j][k][1] = 0;
} else {
dp[i][j][k][2] = 0;
}
//cout << i << ' ' << j << ' ' << k << ": " << dp[i][j][k][0] << ' ' << dp[i][j][k][1] << ' ' << dp[i][j][k][2] << '\n';
continue;
}
dp[i][j][k][0] = INT32_MAX; dp[i][j][k][1] = INT32_MAX; dp[i][j][k][2] = INT32_MAX;
if (i != 0) {
rpos = max((ll)0, j - prefix[rvec[i - 1]][1]) + max((ll)0, k - prefix[rvec[i - 1]][2]);
if (dp[i - 1][j][k][1] != -1) {
dp[i][j][k][0] = min(dp[i][j][k][0], rpos + dp[i - 1][j][k][1]);
}
if (dp[i - 1][j][k][2] != -1) {
dp[i][j][k][0] = min(dp[i][j][k][0], rpos + dp[i - 1][j][k][2]);
}
}
if (j != 0) {
gpos = max((ll)0, i - prefix[gvec[j -1]][0]) + max((ll)0, k - prefix[gvec[j - 1]][2]);
if (dp[i][j - 1][k][0] != -1) {
dp[i][j][k][1] = min(dp[i][j][k][1], gpos + dp[i][j - 1][k][0]);
}
if (dp[i][j - 1][k][2] != -1) {
dp[i][j][k][1] = min(dp[i][j][k][1], gpos + dp[i][j - 1][k][2]);
}
}
if (k != 0) {
ypos = max((ll)0, j - prefix[yvec[k -1]][1]) + max((ll)0, i - prefix[yvec[k - 1]][0]);
if (dp[i][j][k - 1][0] != -1) {
dp[i][j][k][2] = min(dp[i][j][k][2], ypos + dp[i][j][k - 1][0]);
}
if (dp[i][j][k - 1][1] != -1) {
dp[i][j][k][2] = min(dp[i][j][k][2], ypos + dp[i][j][k - 1][1]);
}
}
//cout << i << ' ' << j << ' ' << k << ": " << dp[i][j][k][0] << ' ' << dp[i][j][k][1] << ' ' << dp[i][j][k][2] << '\n';
}
}
}
ll ans = INT32_MAX;
ans = min(ans, min(dp[rsum][gsum][ysum][0], min(dp[rsum][gsum][ysum][1], dp[rsum][gsum][ysum][2])));
cout << ans;
}
