#include "bits/stdc++.h"
using namespace std;
#define ll long long
#define ln '\n'
const int INF = 1e9;
struct Seg{
int n;
vector<int> seg;
void init(int n_){
n = n_;
seg.assign(2*n, 0);
}
void upd(int p, int delta){
for (seg[p += n] += delta; p > 1; p /= 2) seg[p/2] = seg[p] + seg[p^1];
}
int sum(int l, int r){
int res = 0;
for (l += n, r += n+1; l < r; l /= 2, r /= 2){
if (l & 1) res += seg[l++];
if (r & 1) res += seg[--r];
}
return res;
}
} seg;
int n, sz[3];
string s;
int dp[405][405][405][4];
bool is_set[405][405][405][4];
vector<int> where[3];
int f(array<int, 4>& x){
// for (int i = 0; i < 4; i++) cout << x[i] << " ";
// cout << ln;
if (is_set[x[0]][x[1]][x[2]][x[3]]) return dp[x[0]][x[1]][x[2]][x[3]];
int ans = INF;
int old_x3 = x[3];
for (int i = 0; i < 3; i++){
if (i == x[3]) continue; // prev moved
if (x[i] == sz[i]) continue; // already at end
// use where[i][x[i]]
seg.upd(where[i][x[i]], 1);
int t = seg.sum(where[i][x[i]]+1, n-1) + where[i][x[i]] - (x[0] + x[1] + x[2]);
x[i]++; x[3] = i;
ans = min(ans, t + (!is_set[x[0]][x[1]][x[2]][x[3]]? f(x): dp[x[0]][x[1]][x[2]][x[3]]));
x[i]--; x[3] = old_x3;
seg.upd(where[i][x[i]], -1);
}
dp[x[0]][x[1]][x[2]][x[3]] = min(ans, INF); // no overflow
is_set[x[0]][x[1]][x[2]][x[3]] = 1;
return dp[x[0]][x[1]][x[2]][x[3]];
}
void solve(){
cin >> n >> s;
seg.init(n);
for (int i = 0; i < n; i++){
if (s[i] == 'R') where[0].push_back(i);
if (s[i] == 'G') where[1].push_back(i);
if (s[i] == 'Y') where[2].push_back(i);
}
for (int i = 0; i < 3; i++) sz[i] = where[i].size();
for (int i = 0; i < 3; i++) is_set[sz[0]][sz[1]][sz[2]][i] = 1;
array<int, 4> xx = {0, 0, 0, 3};
int ans = f(xx);
cout << (ans == INF? -1: ans) << ln;
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
// int TT; cin >> TT;
// while (TT--) {solve();}
solve();
}
