#include <bits/stdc++.h>
#define ll long long
#define pii pair<int, int>
#define xx first
#define yy second
#define pb push_back
#define MAXN 410
#define MAXX 1000000010LL
using namespace std;
int n;
ll cnt1, cnt2, cnt3, cnt[5][MAXN][5];
string s;
ll koliko(int a, int i, int j, int k)
{
if (a==1)
return max(0LL, cnt[1][i][2]-j)+max(0LL, cnt[1][i][3]-k);
else if (a==2)
return max(0LL, cnt[2][j][1]-i)+max(0LL, cnt[2][j][3]-k);
else
return max(0LL, cnt[3][k][1]-i)+max(0LL, cnt[3][k][2]-j);
}
int main(){
///R=1; G=2; Y=3;
cin>>n>>s;
for (int i=0; i<n; i++)
{
if (s[i]=='R') { cnt1++; }
else if (s[i]=='G') { cnt2++; }
else {cnt3++; }
if (s[i]=='R') { cnt[1][cnt1][2]=cnt2; cnt[1][cnt1][3]=cnt3; }
else if (s[i]=='G') { cnt[2][cnt2][1]=cnt1; cnt[2][cnt2][3]=cnt3; }
else { cnt[3][cnt3][1]=cnt1; cnt[3][cnt3][2]=cnt2; }
}
/*for (int i=0; i<=cnt1; i++) cout<<cnt[1][i][2]<<" "<<cnt[1][i][3]<<endl;
cout<<endl;
for (int i=0; i<=cnt2; i++) cout<<cnt[2][i][1]<<" "<<cnt[2][i][3]<<endl;
cout<<endl;
for (int i=0; i<=cnt3; i++) cout<<cnt[3][i][1]<<" "<<cnt[3][i][2]<<endl;
cout<<endl;*/
ll dp[3+2][cnt1+2][cnt2+2][cnt3+2];
for (int i=0; i<=cnt1; i++) for (int j=0; j<=cnt2; j++) for (int k=0; k<=cnt3; k++) dp[1][i][j][k]=dp[2][i][j][k]=dp[3][i][j][k]=MAXX;
dp[1][1][0][0]=cnt[1][1][2]+cnt[1][1][3]; //cout<<dp[1][1][0][0]<<endl;
dp[2][0][1][0]=cnt[2][1][1]+cnt[2][1][3]; //cout<<dp[2][0][1][0]<<endl;
dp[3][0][0][1]=cnt[3][1][1]+cnt[3][1][2]; //cout<<dp[3][0][0][1]<<endl;
//cout<<endl;
for (int sum=2; sum<=n; sum++)
{
for (int i=0; i<=min((ll)sum, cnt1); i++)
{
for (int j=0; j<=min(cnt2, (ll)(sum-i)); j++)
{
int k=sum-i-j;
if (k>cnt3) continue;
if (i==0) dp[1][i][j][k]=MAXX;
else dp[1][i][j][k]=min(dp[2][i-1][j][k]+koliko(1, i, j, k), dp[3][i-1][j][k]+koliko(1, i, j, k));
if (j==0) dp[2][i][j][k]=MAXX;
else dp[2][i][j][k]=min(dp[1][i][j-1][k]+koliko(2, i, j, k), dp[3][i][j-1][k]+koliko(2, i, j, k));
if (k==0) dp[3][i][j][k]=MAXX;
else dp[3][i][j][k]=min(dp[1][i][j][k-1]+koliko(3, i, j, k), dp[2][i][j][k-1]+koliko(3, i, j, k));
//cout<<i<<" "<<j<<" "<<k<<":"<<endl;
//cout<<dp[1][i][j][k]<<" "<<dp[2][i][j][k]<<" "<<dp[3][i][j][k]<<endl;
}
}
}
ll ress=min(dp[1][cnt1][cnt2][cnt3], min(dp[2][cnt1][cnt2][cnt3], dp[3][cnt1][cnt2][cnt3]));
if (ress>=MAXX) cout<<-1;
else cout<<ress;
return 0;
}
