#include "bits/stdc++.h"
using namespace std;
using i64 = long long;
using u64 = unsigned long long;
using i32 = int;
using u32 = unsigned;
using i16 = short;
using u16 = unsigned short;
using ld = long double;
using ii = pair<int, int>;
const int N = 401, INF = 1e9;
int dp[N][N][N][3];
void solve() {
int n, r, g, y;
string s, p = "RGY";
vector<int> pos[3], cnt[3][3];
cin >> n >> s;
for(int i = 0; i < n; ++i) {
for(int j = 0; j < 3; ++j) {
if(s[i] == p[j])
pos[j].push_back(i);
}
}
r = pos[0].size();
g = pos[1].size();
y = pos[2].size();
for(int i = 0; i <= r; ++i)
for(int j = 0; j <= g; ++j)
for(int k = 0; k <= y; ++k)
for(int l = 0; l < 3; ++l)
dp[i][j][k][l] = INF;
for(int i = 0; i < 3; ++i) {
int n = pos[i].size();
for(int j = 0; j < 3; ++j) {
cnt[i][j].resize(n);
for(int k = 0; k < n; ++k)
cnt[i][j][k] = lower_bound(pos[j].begin(), pos[j].end(), pos[i][k]) - pos[j].begin();
}
}
for(int l = 0; l < 3; ++l) dp[0][0][0][l] = 0;
for(int i = 0; i <= r; ++i) {
for(int j = 0; j <= g; ++j) {
for(int k = 0; k <= y; ++k) {
if(i > 0) {
int moves = max(0, cnt[0][1][i - 1] - j) + max(0, cnt[0][2][i - 1] - k);
dp[i][j][k][0] = min(dp[i][j][k][0], min(dp[i - 1][j][k][1], dp[i - 1][j][k][2]) + moves);
}
if(j > 0) {
int moves = max(0, cnt[1][0][j - 1] - i) + max(0, cnt[1][2][j - 1] - k);
dp[i][j][k][1] = min(dp[i][j][k][1], min(dp[i][j - 1][k][0], dp[i][j - 1][k][2]) + moves);
}
if(k > 0) {
int moves = max(0, cnt[2][0][k - 1] - i) + max(0, cnt[2][1][k - 1] - j);
dp[i][j][k][2] = min(dp[i][j][k][2], min(dp[i][j][k - 1][0], dp[i][j][k - 1][1]) + moves);
}
}
}
}
int ans = min({dp[r][g][y][0], dp[r][g][y][1], dp[r][g][y][2]});
if(ans == INF) ans = -1;
cout << ans << '\n';
}
int main() {
ios_base :: sync_with_stdio(false);
cin.tie(0);
int t = 1;
//cin >> t;
while(t--) solve();
return 0;
}
