// In the name of God
#include<bits/stdc++.h>
using namespace std;
#define pb push_back
#define fi first
#define se second
#define mp make_pair
typedef long long ll;
const int N = 400 + 5;
int n;
string s;
int dp[N][N][N][3], cnt[N][3];
vector<int> vec[3];
void solve() {
cin >> n >> s;
for (int i = 0; i < n; i++) {
for (int j = 0; j < 3 && i; j++)
cnt[i][j] = cnt[i - 1][j];
if (s[i] == 'R') {
cnt[i][0]++;
vec[0].pb(i);
}
else if (s[i] == 'G') {
cnt[i][1]++;
vec[1].pb(i);
}
else {
cnt[i][2]++;
vec[2].pb(i);
}
}
for (int r = 0; r <= cnt[n - 1][0]; r++) {
for (int g = 0; g <= cnt[n - 1][1]; g++) {
for (int y = 0; y <= cnt[n - 1][2]; y++) {
for (int i = 0; i < 3; i++) {
dp[r][g][y][i] = 1e9;
if ((i == 0 && r == 0) || (i == 1 && g == 0) || (i == 2 && y == 0)) {
if (r == 0 && g == 0 && y == 0)
dp[r][g][y][i] = 0;
continue;
}
int r2 = r - (i == 0), g2 = g - (i == 1), y2 = y - (i == 2);
int k = vec[i][(i == 0 ? r : (i == 1 ? g : y)) - 1];
int d = (i != 0) * max(0, cnt[k][0] - r) + (i != 1) * max(0, cnt[k][1] - g) + (i != 2) * max(0, cnt[k][2] - y);
for (int j = 0; j < 3; j++) {
if (i == j)
continue;
dp[r][g][y][i] = min(dp[r][g][y][i], dp[r2][g2][y2][j] + d);
}
}
}
}
}
int ans = 1e9;
for (int i = 0; i < 3; i++)
ans = min(ans, dp[cnt[n - 1][0]][cnt[n - 1][1]][cnt[n - 1][2]][i]);
cout << (ans < 1e9 ? ans : -1) << endl;
}
int32_t main() {
ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
int tc = 1; // cin >> tc;
while (tc--) {
solve();
}
return 0;
}
