#include <bits/stdc++.h>
#include <experimental/random>
#include <random>
//#include <ext/pb_ds/assoc_container.hpp>
//using namespace __gnu_pbds;
using namespace std;
using ld = long double;
using ll = long long;
const ll INF = 1e18, MOD = 1e9 + 7;
void solve();
signed main() {
#ifdef LOCAL
freopen("input.txt", "r", stdin);
freopen("output.txt", "w", stdout);
#endif
ios::sync_with_stdio(false);
cin.tie(0);
int q = 1;
//cin >> q;
while (q--) {
solve();
}
}
void solve() {
ll n;
cin >> n;
string s;
cin >> s;
vector<vector<ll>> p(3, vector<ll>(n + 1));
vector<vector<ll>> t(3);
for (int i = 0; i < n; i++) {
p[0][i + 1] = p[0][i], p[1][i + 1] = p[1][i], p[2][i + 1] = p[2][i];
if (s[i] == 'R') {
p[0][i + 1]++;
t[0].push_back(i);
} else if (s[i] == 'G') {
p[1][i + 1]++;
t[1].push_back(i);
} else {
p[2][i + 1]++;
t[2].push_back(i);
}
}
ll n1 = (int) t[0].size(), n2 = (int) t[1].size(), n3 = (int) t[2].size();
vector<vector<vector<vector<ll>>>> dp(3, vector<vector<vector<ll>>>(n1 + 1, vector<vector<ll>>(n2 + 1, vector<ll>(n3 + 1, INF))));
if (n1 > 0) {
dp[0][1][0][0] = 0;
}
if (n2 > 0) {
dp[1][0][1][0] = 0;
}
if (n3 > 0) {
dp[2][0][0][1] = 0;
}
for (int i = 0; i <= n1; i++) {
for (int j = 0; j <= n2; j++) {
for (int k = 0; k <= n3; k++) {
if (i + j + k < 2) continue;
if (i != 0) {
ll cnt = max(0ll, j - p[1][t[0][i - 1]]) +
max(0ll, k - p[2][t[0][i - 1]]);
dp[0][i][j][k] = min(dp[1][i - 1][j][k], dp[2][i - 1][j][k]) + cnt;
}
if (j != 0) {
ll cnt = max(0ll, i - p[0][t[1][j - 1]]) +
max(0ll, k - p[2][t[1][j - 1]]);
dp[1][i][j][k] = min(dp[0][i][j - 1][k], dp[2][i][j - 1][k]) + cnt;
}
if (k != 0) {
ll cnt = max(0ll, i - p[0][t[2][k - 1]]) +
max(0ll, j - p[1][t[2][k - 1]]);
dp[2][i][j][k] = min(dp[0][i][j][k - 1], dp[1][i][j][k - 1]) + cnt;
}
}
}
}
ll ans = min({dp[0][n1][n2][n3], dp[1][n1][n2][n3], dp[2][n1][n2][n3]});
if (ans >= INF) {
cout << -1;
} else {
cout << ans;
}
}
