#include <bits/stdc++.h>
using namespace std;
//#define int long long
//#define all(x) x.begin(),x.end()
//#define rall(x) x.rbegin(),x.rend()
//#define ff first
//#define ss second
//#define pb push_back
//template<class T, class U> inline bool chmin(T& a, const U& b) { if (a > b) { a = b; return true; } return false; }
//template<class T, class U> inline bool chmax(T& a, const U& b) { if (a < b) { a = b; return true; } return false; }
//mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
//#define rnd(l, r) uniform_int_distribution <int> (l, r)(rng)
const int inf = 1e6, mod = 1e9 + 7;
void solve(){
int n; cin >> n;
string s; cin >> s;
int x = 0, y = 0, z = 0;
vector<int> a, b, c;
for(int i = 0; i < n; i ++ ) {
if(s[i] == 'R')x ++, a.push_back(i);
if(s[i] == 'G')y ++, b.push_back(i);
if(s[i] == 'Y')z ++, c.push_back(i);
};
a.push_back(n);
b.push_back(n);
c.push_back(n);
int dp[x+1][y+1][z+1][3];
memset(dp, inf, sizeof(dp));
for(int i = 0; i < 3; i++)dp[0][0][0][i] = 0;
for(int i = 0; i <= x; i ++ ) {
for(int j = 0; j <= y; j ++ ) {
for(int k = 0; k <= z; k ++ ) {
for(int la = 0; la < 3; la ++ ) {
// i + 1;
if(la != 0 && i < x) {
int lwb = lower_bound(b.begin(), b.end(), a[i]) - b.begin();
int lwc = lower_bound(c.begin(), c.end(), a[i]) - c.begin();
int cost = max(0, lwb - j) + max(0, lwc - k);
dp[i+1][j][k][0] = min(dp[i+1][j][k][0], cost + dp[i][j][k][la]);
};
// j + 1;
if(la != 1 && j < y) {
int lwa = lower_bound(a.begin(), a.end(), b[j]) - a.begin();
int lwc = lower_bound(c.begin(), c.end(), b[j]) - c.begin();
int cost = max(0, lwa - i) + max(0, lwc - k);
dp[i][j+1][k][1] = min(dp[i][j+1][k][1], cost + dp[i][j][k][la]);
};
// k + 1;
if(la != 2 && k < z) {
int lwb = lower_bound(b.begin(), b.end(), c[k]) - b.begin();
int lwa = lower_bound(a.begin(), a.end(), c[k]) - a.begin();
int cost = max(0, lwb - j) + max(0, lwa - i);
dp[i][j][k+1][2] = min(dp[i][j][k+1][2], cost + dp[i][j][k][la]);
};
//cout << i << ' ' << j << ' ' << k << ' '<< la << ' ' << dp[i][j][k][la] << '\n';
};
};
};
};
int ans = inf;
for(int i = 0; i < 3; i++)
ans = min(ans, dp[x][y][z][i]);
if(ans == inf)cout << -1 << '\n';
else cout << ans << '\n';
}
signed main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);cout.tie(nullptr);
int tt = 1;
//cin >> tt;
while(tt--){
solve();
};
}
