#include <bits/stdc++.h>
using namespace std;
#ifdef ngu
#include "debug.h"
#else
#define debug(...) 42
#endif
const int N = 405, INF = (int) 1e9;
int n;
char s[N];
int a[N], sz[N];
int cost[3][N][N];
vector<int> pos[3];
template<class A, class B>
bool mini(A &a, const B &b) {
return a > b ? a = b, true : false;
}
signed main() {
ios::sync_with_stdio(false);
cin.tie(0);
#ifdef ngu
freopen("test.inp", "r", stdin);
freopen("test.out", "w", stdout);
#endif
cin >> n;
for (int i = 1; i <= n; ++i) {
cin >> s[i];
a[i] = (s[i] == 'Y' ? 2 : (s[i] == 'G'));
pos[a[i]].emplace_back(i);
sz[a[i]]++;
}
for (int c = 0; c < 3; ++c) {
for (int i = 0; i <= sz[c]; ++i) {
for (int j = 1; j <= n; ++j) {
for (int k = i; k < sz[c]; ++k) {
cost[c][i][j] += j > pos[c][k];
}
}
}
}
vector<vector<vector<vector<int>>>> dp(sz[0] + 1, vector<vector<vector<int>>>(sz[1] + 1, vector<vector<int>>(sz[2] + 1, vector<int>(3, INF))));
for (int c = 0; c < 3; ++c) {
dp[0][0][0][c] = 0;
}
for (int i = 0; i <= sz[0]; ++i) {
for (int j = 0; j <= sz[1]; ++j) {
for (int k = 0; k <= sz[2]; ++k) {
for (int c = 0; c < 3; ++c) {
if (dp[i][j][k][c] != INF) {
auto calc = [&](int idx) {
return cost[0][i][idx] + cost[1][j][idx] + cost[2][k][idx];
};
if (c != 0 && i + 1 <= sz[0]) {
mini(dp[i + 1][j][k][0], dp[i][j][k][c] + calc(pos[0][i]));
}
if (c != 1 && j + 1 <= sz[1]) {
mini(dp[i][j + 1][k][1], dp[i][j][k][c] + calc(pos[1][j]));
}
if (c != 2 && k + 1 <= sz[2]) {
mini(dp[i][j][k + 1][2], dp[i][j][k][c] + calc(pos[2][k]));
}
}
}
}
}
}
int res = INF;
for (int c = 0; c < 3; ++c) {
res = min(res, dp[sz[0]][sz[1]][sz[2]][c]);
}
cout << (res == INF ? -1 : res);
}
