#include "bits/stdc++.h"
using namespace std;
#define for_(i, s, e) for (int i = s; i < (int) e; i++)
#define for__(i, s, e) for (ll i = s; i < e; i++)
typedef long long ll;
typedef vector<int> vi;
typedef array<int, 2> ii;
#define endl '\n'
int main() {
#ifdef mlocal
freopen("test.in", "r", stdin);
#endif
ios_base::sync_with_stdio(false);
cin.tie(0);
int n; string s; cin >> n >> s;
vi nums(n);
vector<vi> cpos(3);
for_(i, 0, n) {
if (s[i] == 'G') nums[i] = 1;
else if (s[i] == 'Y') nums[i] = 2;
cpos[nums[i]].push_back(i);
}
const int INF = 1e9;
int dp[n+1][n][n][3], t[3], ct[3];
memset(dp, 0x3f, sizeof(dp));
for_(i, 0, 3) ct[i] = cpos[i].size();
int ans;
for (int i = n; i >= 0; i--) for (t[0] = 0; t[0] < min(ct[0], i) + 1; t[0]++) for (t[1] = 0; t[1] < min(ct[1], i-t[0]) + 1; t[1]++) {
t[2] = i-t[0]-t[1];
if (t[2] > ct[2]) continue;
for_(last, 0, 3) {
ans = INF;
if (i == n) {
ans = (t[0] == ct[0] and t[1] == ct[1] and t[2] == ct[2]) ? 0 : INF;
}
else for_(c, 0, 3) if (c != last and ct[c] > t[c]) {
ans = min(ans, dp[i+1][t[0]+(c==0)][t[1]+(c==1)][c] + max(i, cpos[c][t[c]]) - i);
}
dp[i][t[0]][t[1]][last] = ans;
}
}
ans = min({dp[0][0][0][1], dp[0][0][0][0]});
cout << (ans >= INF ? -1 : ans) << endl;
return 0;
}
