// 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 = 1e7;
const int N = 400 + 1;
int dp[N][N][N][3];
int idx[3][N];
int ns[3];
int pos[3][N];
// vi pos[3];
int go(int i, int j, int k, int c) {
bool place0 = i < ns[0];
bool place1 = j < ns[1];
bool place2 = k < ns[2];
if (!place0 && !place1 && !place2)
return 0;
if (dp[i][j][k][c] != -1)
return dp[i][j][k][c];
int first = oo;
if (place0) first = min(first, pos[0][i]);
if (place1) first = min(first, pos[1][j]);
if (place2) first = min(first, pos[2][k]);
place0 &= c != 0;
place1 &= c != 1;
place2 &= c != 2;
int p = i + j + k;
int ans = oo;
if (place0) {
int to = pos[0][i];
int shift = max(0, j - idx[1][to]) + max(0, k - idx[2][to]);
to += shift;
ans = min(ans, abs(p - to) + go(i + 1, j, k, 0));
}
if (place1) {
int to = pos[1][j];
int shift = max(0, i - idx[0][to]) + max(0, k - idx[2][to]);
to += shift;
ans = min(ans, abs(p - to) + go(i, j + 1, k, 1));
}
if (place2) {
int to = pos[2][k];
int shift = max(0, j - idx[1][to]) + max(0, i - idx[0][to]);
to += shift;
ans = min(ans, abs(p - to) + 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;
ns[0] = ns[1] = ns[2] = 0;
for (int i = 0; i < n; i++) {
if (s[i] == 'R') pos[0][ns[0]++] = i;
if (s[i] == 'G') pos[1][ns[1]++] = i;
if (s[i] == 'Y') pos[2][ns[2]++] = i;
for (int j = 0; j < 3; j++)
idx[j][i] = ns[j];
}
for (int i = ns[0]; i >= 0; i--)
for (int j = ns[1]; j >= 0; j--)
for (int k = ns[2]; k >= 0; k--)
for (int c = 0; c < 3; c++)
go(i, j, k, c);
int ans = min({go(0, 0, 0, 0), go(0, 0, 0, 1), go(0, 0, 0, 2)});
if (ans >= oo)
ans = -1;
cout << ans << "\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;
}
