#include<bits/stdc++.h>
using namespace std;
#define ll int
#define pii pair<ll,ll>
#define pb push_back
#define fi first
#define se second
const ll N = 403;
const ll INF = 1e9;
ll n,r,g,b;
string s;
vector<ll> pos[5];
ll dp[N][N][N][3];
int main() {
cin >> n >> s;
s = '!' + s;
for(int i=1; i<=n; i++) {
if(s[i] == 'R') {
r++;
pos[0].pb(i);
}
else if(s[i] == 'G') {
g++;
pos[1].pb(i);
}
else if(s[i] == 'Y') {
b++;
pos[2].pb(i);
}
}
if(r > (n+1)/2 || g > (n+1)/2 || b > (n+1)/2) {
cout << -1 << endl;
return 0;
}
for(int i=0; i<=r; i++) {
for(int j=0; j<=g; j++) {
for(int k=0; k<=b; k++) {
for(int l=0; l<=2; l++) dp[i][j][k][l] = INF;
}
}
}
dp[0][0][0][0] = 0;
dp[0][0][0][1] = 0;
dp[0][0][0][2] = 0;
for(int i=0; i<=r; i++) {
for(int j=0; j<=g; j++) {
for(int k=0; k<=b; k++) {
if(i) dp[i][j][k][0] = min(dp[i-1][j][k][1],dp[i-1][j][k][2]) + abs(i+j+k-pos[0][i-1]);
if(j) dp[i][j][k][1] = min(dp[i][j-1][k][0],dp[i][j-1][k][2]) + abs(i+j+k-pos[1][j-1]);
if(k) dp[i][j][k][2] = min(dp[i][j][k-1][0],dp[i][j][k-1][1]) + abs(i+j+k-pos[2][k-1]);
}
}
}
ll ans = min({dp[r][g][b][0],dp[r][g][b][1],dp[r][g][b][2]});
if(ans >= INF) cout << -1 << endl;
else cout << ans/2 << endl;
}
