#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define pb push_back
#define fi first
#define se second
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int n;
cin >> n;
string s;
cin >> s;
vector<int> yy, gg, rr;
for (int i = 0; i < n; i++) {
if (s[i] == 'Y') yy.pb(i);
else if (s[i] == 'G') gg.pb(i);
else rr.pb(i);
}
int iy = yy.size(), ig = gg.size(), ir = rr.size();
vector<vector<vector<vector<int>>>> dp(iy + 1, vector<vector<vector<int>>>(ig + 1, vector<vector<int>>(ir + 1, vector<int>(3, 1e9))));
dp[0][0][0][0] = 0;
dp[0][0][0][1] = 0;
dp[0][0][0][2] = 0;
for (int y = 0; y <= yy.size(); y++) {
for (int g = 0; g <= gg.size(); g++) {
for (int r = 0; r <= rr.size(); r++) {
int w = y + g + r;
if (y != 0) {
int przed = min(g, int(lower_bound(gg.begin(), gg.end(), yy[y - 1]) - gg.begin()))
+ min(r, int(lower_bound(rr.begin(), rr.end(), yy[y - 1]) - rr.begin()));
int po = w - 1 - przed;
int koszt = max(0, yy[y - 1] - (y - 1) - przed);
dp[y][g][r][0] = min(dp[y-1][g][r][1], dp[y-1][g][r][2]) + koszt;
}
if (g != 0) {
int przed = min(y, int(lower_bound(yy.begin(), yy.end(), gg[g - 1]) - yy.begin()))
+ min(r, int(lower_bound(rr.begin(), rr.end(), gg[g - 1]) - rr.begin()));
int po = w - 1 - przed;
int koszt = max(0, gg[g - 1] - (g - 1) - przed);
dp[y][g][r][1] = min(dp[y][g-1][r][0], dp[y][g-1][r][2]) + koszt;
}
if (r != 0) {
int przed = min(g, int(lower_bound(gg.begin(), gg.end(), rr[r - 1]) - gg.begin()))
+ min(y, int(lower_bound(yy.begin(), yy.end(), rr[r - 1]) - yy.begin()));
int po = w - 1 - przed;
int koszt = max(0, rr[r - 1] - (r - 1) - przed);
dp[y][g][r][2] = min(dp[y][g][r-1][0], dp[y][g][r-1][1]) + koszt;
}
}
}
}
int wyn = min(dp[iy][ig][ir][0], min(dp[iy][ig][ir][1], dp[iy][ig][ir][2]));
if (wyn == 1e9) cout << -1;
else cout << wyn;
}
