#include <bits/stdc++.h>
using namespace std;
#define int long long
#define double long double
#define FOR(i, l, r, d) for(int i=(l); i<=(r); i+=(d))
#define szof(x) ((int)(x).size())
#define vi vector<int>
#define pii pair<int, int>
#define F first
#define S second
#define pb push_back
#define eb emplace_back
#define mkp make_pair
const int INF = INT_MAX;
const int LNF = INF*INF;
const int MOD = 1000000007;
const int mod = 998244353;
//#pragma GCC optimize("Ofast")
//#pragma GCC optimize("unroll-loops")
const int MAX = 410;
int n;
char arr[MAX];
int R, G, Y;
int rp[MAX], gp[MAX], yp[MAX];
vector<vector<vector<vi>>> dp;
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
cin>>n;
FOR(i, 1, n, 1){
cin>>arr[i];
if(arr[i] == 'R') rp[++R] = i;
if(arr[i] == 'G') gp[++G] = i;
if(arr[i] == 'Y') yp[++Y] = i;
}
if(max({R, G, Y}) > (n + 1) / 2){
cout<<-1<<'\n';
return 0;
}
dp.resize(3);
for(auto& i : dp){
i.resize(R+1);
for(auto& j : i){
j.resize(G+1);
for(auto& k : j){
k.resize(Y+1);
for(int& l : k){
l = INF;
}
}
}
}
dp[0][0][0][0] = dp[1][0][0][0] = dp[2][0][0][0] = 0;
FOR(i, 0, R, 1){
FOR(j, 0, G, 1){
FOR(k, 0, Y, 1){
int pos = i + j + k;
int dr = abs(rp[i] - pos);
int dg = abs(gp[j] - pos);
int dy = abs(yp[k] - pos);
if(i > 0){
dp[0][i][j][k] = min({dp[0][i][j][k], dp[1][i-1][j][k] + dr, dp[2][i-1][j][k] + dr});
}
if(j > 0){
dp[1][i][j][k] = min({dp[1][i][j][k], dp[0][i][j-1][k] + dg, dp[2][i][j-1][k] + dg});
}
if(k > 0){
dp[2][i][j][k] = min({dp[2][i][j][k], dp[0][i][j][k-1] + dy, dp[1][i][j][k-1] + dy});
}
}
}
}
int res = min({dp[0][R][G][Y], dp[1][R][G][Y], dp[2][R][G][Y]}) / 2;
cout<<res<<'\n';
return 0;
}
