#include <bits/stdc++.h>
//ako ovaj vaso daso misli da me pobedjuje.....
using namespace std;
#define int long long
#define f first
#define s second
const int N = 410;
const int inf = 1e9 + 69;
int dp[N][N][N][3], n;
string s;
vector<int> pos[3];
signed main() {
ios::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
int tt = 1;
//cin >> tt;
while (tt--) {
cin >> n >> s;
int a = 0, b = 0, c = 0;
for (int i = 0;i < n;i++) {
if (s[i] == 'R')
s[i] = 'A';
else if (s[i] == 'G')
s[i] = 'B';
else
s[i] = 'C';
a += (s[i] == 'A');
b += (s[i] == 'B');
c += (s[i] == 'C');
pos[s[i] - 'A'].push_back(i);
}
for (int i = 0;i <= a;i++) {
for (int j = 0;j <= b;j++) {
for (int k = 0;k <= c;k++)
dp[i][j][k][0] = dp[i][j][k][1] = dp[i][j][k][2] = inf;
}
}
dp[0][0][0][0] = dp[0][0][0][1] = dp[0][0][0][2] = 0;
for (int i = 0;i <= a;i++) {
for (int j = 0;j <= b;j++) {
for (int k = 0;k <= c;k++) {
int p = i + j + k - 1;
if (i > 0) {
dp[i][j][k][0] = min(dp[i - 1][j][k][1], dp[i - 1][j][k][2]) + abs(pos[0][i - 1] - p);
}
if (j > 0) {
dp[i][j][k][1] = min(dp[i][j - 1][k][0], dp[i][j - 1][k][2]) + abs(pos[1][j - 1] - p);
}
if (k > 0) {
dp[i][j][k][2] = min(dp[i][j][k - 1][1], dp[i][j][k - 1][0]) + abs(pos[2][k - 1] - p);
}
}
}
}
int ans = inf;
for (int i = 0;i < 3;i++)
ans = min(ans, dp[a][b][c][i]);
if (ans >= inf)
ans = -2;
cout << ans / 2;
}
return 0;
}
