#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
#include "debug.h"
#else
#define debug(...) 42
#endif
const int N = 405;
int n;
string s;
int sf[3][N][N], dp[N][N][N][3];
int conv(char c) {
return c == 'R' ? 0 : (c == 'G' ? 1 : 2);
}
int main() {
ios::sync_with_stdio(false); cin.tie(nullptr);
cin >> n >> s;
array<int, 3> cnt{};
array<vector<int>, 3> pos;
for (int i = 0; i < n; ++i) {
int c = conv(s[i]);
++cnt[c];
pos[c].push_back(i);
}
for (int i = 0; i < 3; ++i) {
for (int j = 1; j <= n; ++j) {
for (int k = cnt[i] - 1; k >= 0; --k) {
sf[i][j][k] = sf[i][j][k + 1] + (pos[i][k] < j);
}
}
}
memset(dp, 0x3f, sizeof(dp));
for (int j = 0; j < 3; ++j) {
dp[0][0][0][j] = 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 c = 0; c < 3; ++c) {
if (dp[i][j][k][c] < 1e9) {
auto cost = [&](int p) {
return sf[0][p][i] + sf[1][p][j] + sf[2][p][k];
};
if (c != 0 && i + 1 <= cnt[0]) {
dp[i + 1][j][k][0] = min(dp[i + 1][j][k][0], dp[i][j][k][c] + cost(pos[0][i]));
}
if (c != 1 && j + 1 <= cnt[1]) {
dp[i][j + 1][k][1] = min(dp[i][j + 1][k][1], dp[i][j][k][c] + cost(pos[1][j]));
}
if (c != 2 && k + 1 <= cnt[2]) {
dp[i][j][k + 1][2] = min(dp[i][j][k + 1][2], dp[i][j][k][c] + cost(pos[2][k]));
}
}
}
}
}
}
int res = *min_element(dp[cnt[0]][cnt[1]][cnt[2]], dp[cnt[0]][cnt[1]][cnt[2]] + 3);
cout << (res > 1e9 ? -1 : res);
return 0;
}
