// I'm a paragraph. Click here to add your own text and edit me. It’s easy.
// Just click “Edit Text” or double click me to add your own content
// and make changes to the font. Feel free to drag and drop me anywhere
// you like on your page. I’m a great place for you to tell a story
// and let your users know a little more about you.
#include <bits/stdc++.h>
using namespace std;
#define filename ""
#define pb push_back
#define mp make_pair
#define eb emplace_back
#define xx first
#define yy second
#define sz(x) ((int) size(x))
#define all(x) begin(x), end(x)
#define rep(i, a, b) for(int i = a; i < b; i++)
#define per(i, a, b) for(int i = a; i >= b; i--)
using ll = long long;
using ld = long double;
using vi = vector<int>;
using vl = vector<ll>;
using pii = pair<int, int>;
const int oo = 1e8;
const int N = 400 + 1;
int dp[N][N][N][3];
vi pos[3];
int go(int i, int j, int k, int c) {
bool place0 = i < sz(pos[0]);
bool place1 = j < sz(pos[1]);
bool place2 = k < sz(pos[2]);
if (!place0 && !place1 && !place2)
return 0;
if (dp[i][j][k][c] != -1)
return dp[i][j][k][c];
place0 &= c != 0;
place1 &= c != 1;
place2 &= c != 2;
int p = i + j + k;
int ans = oo;
if (place0) ans = min(ans, abs(p - pos[0][i]) + go(i + 1, j, k, 0));
if (place1) ans = min(ans, abs(p - pos[1][j]) + go(i, j + 1, k, 1));
if (place2) ans = min(ans, abs(p - pos[2][k]) + go(i, j, k + 1, 2));
return dp[i][j][k][c] = ans;
}
void solve() {
memset(dp, -1, sizeof(dp));
int n;
string s;
cin >> n >> s;
for (int i = 0; i < n; i++) {
if (s[i] == 'R') pos[0].push_back(i);
if (s[i] == 'G') pos[1].push_back(i);
if (s[i] == 'Y') pos[2].push_back(i);
}
int ans = min(go(0, 0, 0, 0), go(0, 0, 0, 1));
if (ans >= oo)
ans = -2;
cout << ans / 2 << "\n";
}
int main() {
cin.tie(0)->sync_with_stdio(0);
cin.exceptions(cin.failbit);
// freopen(filename ".in", "r", stdin); freopen(filename ".out", "w", stdout);
// int t; cin >> t; while (t--)
solve();
return 0;
}
