This submission is migrated from previous version of oj.uz, which used different machine for grading. This submission may have different result if resubmitted.
#include <bits/stdc++.h>
#define ll long long
using namespace std;
int n,m,k,x,y,z;
int val['z'];
char ar[405], tar[405];
int cal() {
    int pos[405];
    for (int i = 1; i <= n; i++) {
        for (int j = 1; j <= n; j++) {
            if (ar[i] == tar[j]) {
                pos[i] = j;
                tar[j] = '#';
                goto nxt;
            }
        }
        return 1e9;
        nxt:;
    }
    int ans = 0;
    for (int i = 1; i <= n; i++) {
        for (int j = 1; j < i; j++) {
            ans += (pos[i] < pos[j]);
        }
    }
    return ans;
}
int main() {
    cin.tie(0), cout.tie(0) -> sync_with_stdio(0);
    cin >> n;
    for (int i = 1; i <= n; i++) cin >> ar[i];
    for (int i = 1; i <= n; i++) tar[i] = (i & 1 ? 'R' : 'G');
    int ans = cal();
    for (int i = 1; i <= n; i++) tar[i] = (i & 1 ? 'G' : 'R');
    ans = min(ans,cal());
    if (ans > 1e8) cout << "-1\n";
    else cout << ans << "\n";
}
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