#include <bits/stdc++.h>
using namespace std;
int num(char lettre) {
if (lettre == 'R') return 0;
if (lettre == 'G') return 1;
return 2;
}
int position(int N, vector<int>& a, vector<int>& count, int start) {
int p1 = start, p2 = (start+1)%2;
int res = 0;
for (int i = 0; i < N; i++) {
if ((a[i] == 0 && count[0] >= count[1]) || (a[i] == 1 && count[0] < count[1])) {
res += abs(p1-i);
p1 += 2;
}
else {
res += abs(p2-i);
p2 += 2;
}
}
return res;
}
int main() {
ios::sync_with_stdio(false);
cin.tie(0);
int N;
cin >> N;
cin.ignore();
vector<int> a(N);
vector<int> count(3, 0);
for (int i = 0; i < N; i++) {
char c;
cin >> c;
a[i] = num(c);
count[num(c)]++;
}
if (*max_element(count.begin(), count.end()) > (N+1)/2) {
cout << -1 << '\n'; return 0;
}
int res = position(N, a, count, 0);
if (count[0] == count[1]) {
res = min(res, position(N, a, count, 1));
}
cout << res/2 << '\n';
return 0;
}
