#include <bits/stdc++.h>
#define FR(i, N) for (int i = 0; i < int(N); i++)
#define all(x) begin(x), end(x)
using namespace std;
using ll = long long;
const int MAXN = 400;
//int dp[MAXN][MAXN][MAXN][3];
int plc[3][MAXN];
int ft[MAXN][3];
int cnt[3];
int INF = (int)(1e8);
int main() {
cin.tie(0);
cin.sync_with_stdio(0);
int N;
cin >> N;
FR(i, N) {
char a;
cin >> a;
int d = (a == 'R' ? 0 : (a == 'G' ? 1 : 2));
cnt[d]++;
plc[d][cnt[d]] = i;
copy(all(cnt), begin(ft[i]));
}
int dp[cnt[0]+1][cnt[1]+1][cnt[2]+1][3];
fill(all(dp[0][0][0]), 0);
for (int i = 0; i <= cnt[0]; i++) {
for (int j = 0; j <= cnt[1]; j++) {
for (int k = 0; k <= cnt[2]; k++) {
for (int z = 0; z <= 2; z++) {
if (i == 0 && j == 0 && k == 0) {
dp[i][j][k][z] = 0;
}
else {
dp[i][j][k][z] = INF;
if (z == 0) {
if (i != 0) {
dp[i][j][k][z] = min(dp[i-1][j][k][1], dp[i-1][j][k][2]) + max(0, ft[plc[0][i]][1] - j) + max(0, ft[plc[0][i]][2] - k);
}
}
else if (z == 1) {
if (j != 0) {
dp[i][j][k][z] = min(dp[i][j-1][k][0], dp[i][j-1][k][2]) + max(0, ft[plc[1][j]][0] - i) + max(0, ft[plc[1][j]][2] - k);
}
}
else {
if (k != 0) {
dp[i][j][k][z] = min(dp[i][j][k-1][0], dp[i][j][k-1][1]) + max(0, ft[plc[2][k]][0] - i) + max(0, ft[plc[2][k]][1] - j);
}
}
// cout << i << " " << j << " " << k << " " << z << ": " <<dp[i][j][k][z] << '\n';
}
}
}
}
}
cout << (*min_element(all(dp[cnt[0]][cnt[1]][cnt[2]])) >= INF ? -1 : *min_element(all(dp[cnt[0]][cnt[1]][cnt[2]]))) << '\n';
return 0;
}
