#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N = 400 + 12, MOD = (int)1e9 + 7;
int n,dp[N][N][N][3];
vector<int> r,g,y;
void test() {
cin >> n;
string s;cin >> s;
for(int i = 0;i < n;i++) {
if(s[i] == 'R') {
r.push_back(i);
} else if(s[i] == 'Y') {
y.push_back(i);
} else {
g.push_back(i);
}
}
int r_ = (int)r.size(),g_ = (int)g.size(),y_ = (int)y.size();
for(int i = 0;i <= n;i++) {
for(int j = 0;j <= n;j++) {
for(int k = 0;k <= n;k++) {
for(int t = 0;t < 3;t++) {
dp[i][j][k][t] = 1e9;
}
}
}
}
for(int i = 0;i < 3;i++) {
dp[0][0][0][i] = 0;
}
for(int i = 0;i <= r_;i++) {
for(int j = 0;j <= g_;j++) {
for(int k = 0;k <= y_;k++) {
for(int t = 0;t < 3;t++){
int all = i + j + k;
if(i < r_ && t != 0) {
dp[i + 1][j][k][0] = min(dp[i + 1][j][k][0],dp[i][j][k][t] + abs(all - r[i]));
}
if(j < g_ && t != 1) {
dp[i][j + 1][k][1] = min(dp[i][j + 1][k][1],dp[i][j][k][t] + abs(all - g[j]));
}
if(k < y_ && t != 2) {
dp[i][j][k + 1][2] = min(dp[i][j][k + 1][2],dp[i][j][k][t] + abs(all - y[k]));
}
}
}
}
}
int res = 1e9;
for(int i = 0;i < 3;i++) {
res = min(res,dp[r_][g_][y_][i]);
}
if(res == 1e9) {
cout << -1;
return;
}
cout << res / 2;
}
int main() {
ios_base::sync_with_stdio(false);cin.tie(0);
int t = 1;
// cin >> t;
while(t--) {
test();
}
}
