#include <bits/stdc++.h>
using namespace std;
#define loop(_i, _a, _b) for(int _i=_a; _i<=_b; ++_i)
const int maxn=401, maxn2=201, inf=1e9;
int n;
int cnt[4][maxn][4];
int val[maxn];
string s;
int main() {
cin.tie(0) -> ios_base::sync_with_stdio(0);
cin >> n >> s;
int a=0, b=0, c=0;
loop(i, 1, n) {
int v;
if(s[i-1] == 'R') v=1;
if(s[i-1] == 'G') v=2;
if(s[i-1] == 'Y') v=3;
val[i]=v;
if(v==1) {
++a;
cnt[1][a][2]=b;
cnt[1][a][3]=c;
}
if(v==2) {
++b;
cnt[2][b][1]=a;
cnt[2][b][3]=c;
}
if(v==3) {
++c;
cnt[3][c][1]=a;
cnt[3][c][2]=b;
}
}
vector<vector<vector<vector<int>>>> dp(a+1, vector<vector<vector<int>>>(b+1, vector<vector<int>>(c+1, vector<int>(4, inf))));
dp[0][0][0][1]=0;
dp[0][0][0][2]=0;
dp[0][0][0][3]=0;
loop(i, 0, a) {
loop(j, 0, b) {
loop(k, 0, c) {
if(i<a) dp[i+1][j][k][1]=min(dp[i+1][j][k][1], min(dp[i][j][k][2], dp[i][j][k][3])+abs(j-cnt[1][i+1][2])+abs(k-cnt[1][i+1][3]));
if(j<b) dp[i][j+1][k][2]=min(dp[i][j+1][k][2], min(dp[i][j][k][1], dp[i][j][k][3])+abs(i-cnt[2][j+1][1])+abs(k-cnt[2][j+1][3]));
if(k<c) dp[i][j][k+1][3]=min(dp[i][j][k+1][3], min(dp[i][j][k][1], dp[i][j][k][2])+abs(i-cnt[3][k+1][1])+abs(j-cnt[3][k+1][2]));
}
}
}
int res = inf;
loop(i, 1, 3) res=min(res, dp[a][b][c][i]);
if(res==inf) cout << "-1";
else cout << res/2;
}
