#include <bits/stdc++.h>
using namespace std;
const int MAXN = 405;
int memo[MAXN][MAXN][MAXN][3], pos[3][MAXN], cnt[3], pref[3][MAXN];
const int inf = 1e9 + 7;
int dp(int c0, int c1, int c2, int fr){
if (c0 == 0 && c1 == 0 && c2 == 0) return 0;
if (memo[c0][c1][c2][fr] != -1) return memo[c0][c1][c2][fr];
int ans = inf;
if (fr == 0){
if (c0 != 0){
int ipos = pos[0][c0], swps = ipos + max(c1 - pref[1][ipos], 0) + max(c2 - pref[2][ipos], 0) - (c0 + c1 + c2);
ans = min(ans, min(dp(c0 - 1, c1, c2, 1), dp(c0 - 1, c1, c2, 2)) + swps);
}
}
else if (fr == 1){
if (c1 != 0){
int ipos = pos[1][c1], swps = ipos + max(c0 - pref[0][ipos], 0) + max(c2 - pref[2][ipos], 0) - (c0 + c1 + c2);
ans = min(ans, min(dp(c0, c1 - 1, c2, 0), dp(c0, c1 - 1, c2, 2)) + swps);
}
}
else{
if (c2 != 0){
int ipos = pos[2][c2], swps = ipos + max(c0 - pref[0][ipos], 0) + max(c1 - pref[1][ipos], 0) - (c0 + c1 + c2);
ans = min(ans, min(dp(c0, c1, c2 - 1, 0), dp(c0, c1, c2 - 1, 1)) + swps);
}
}
return memo[c0][c1][c2][fr] = ans;
}
int main(){
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
int nums; cin >> nums;
for (int i = 1; i <= nums; i++){
char ch; cin >> ch;
int v;
if (ch == 'R') v = 0;
else if (ch == 'Y') v = 1;
else v = 2;
cnt[v]++;
pos[v][cnt[v]] = i;
for (int j = 0; j < 3; j++) pref[j][i] = pref[j][i - 1];
pref[v][i]++;
}
memset(memo, -1, sizeof(memo));
int ans = inf;
for (int j = 0; j < 3; j++) ans = min(ans, dp(cnt[0], cnt[1], cnt[2], j));
cout << ((ans == inf) ? -1 : ans);
}
