#include <bits/stdc++.h>
using namespace std;
#define endl '\n'
#define ll long long
#define all(x) (x).begin(), (x).end()
const int mxn = 402;
int dp[mxn][mxn][mxn][3], sum[mxn][3], pos[mxn][3], cnt[3];
int main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
int n;
cin >> n;
string s;
cin >> s;
s = "." + s;
for (int i = 1; i <= n; i++) {
if (s[i] == 'R') pos[++cnt[0]][0] = i;
if (s[i] == 'G') pos[++cnt[1]][1] = i;
if (s[i] == 'Y') pos[++cnt[2]][2] = i;
for (int j = 0; j < 3; j++) sum[i][j] = cnt[j];
}
for (int j = 0; j < 3; j++) dp[0][0][0][j] = 0;
for (int i = 1; i <= n; i++) {
for (int r = 0; r <= min(i, cnt[0]); r++) {
for (int g = 0; g + r <= i && g <= cnt[1]; g++) {
int y = i - (r + g);
if (y > cnt[2]) continue;
if (r) dp[i][r][g][0] = min(dp[i - 1][r - 1][g][1], dp[i - 1][r - 1][g][2]) + max(0, g - sum[pos[r][0]][1]) + max(0, y - sum[pos[r][0]][2]);
else dp[i][r][g][0] = 1e9;
if (g) dp[i][r][g][1] = min(dp[i - 1][r][g - 1][0], dp[i - 1][r][g - 1][2]) + max(0, r - sum[pos[g][1]][0]) + max(0, y - sum[pos[g][1]][2]);
else dp[i][r][g][1] = 1e9;
if (y) dp[i][r][g][2] = min(dp[i - 1][r][g][0], dp[i - 1][r][g][1]) + max(0, r - sum[pos[y][2]][0] + max(0, g - sum[pos[y][2]][1]));
else dp[i][r][g][2] = 1e9;
}
}
}
int ans = 1e9;
for (int j = 0; j < 3; j++) ans = min(ans, dp[n][cnt[0]][cnt[1]][j]);
cout << (ans == 1e9 ? -1 : ans);
}
