#include <bits/stdc++.h>
#define ll long long
#define rep(a,b,c) for(ll a=b; a<c; a++)
#define repr(a,b,c) for(ll a=b-1; a>c-1; a--)
using namespace std;
int main(){
ll n;
string s, s2, s3;
cin>>n>>s;
s2=s;
ll mn=1e9;
sort(s2.begin(),s2.end());
do{
bool ans=true;
rep(i,0,n-1){
if(s2[i]==s2[i+1]) ans=false;
}
if(ans){
s3=s;
ll sum=0;
rep(i,0,n){
if(s2[i]==s3[i]) continue;
rep(j,i,n){
if(s3[j]==s2[i]){
repr(k,j,i) swap(s3[k+1],s3[k]);
sum+=(j-i);
break;
}
}
}
mn=min(mn,sum);
}
}while(next_permutation(s2.begin(),s2.end()));
if(mn<1e9) cout<<mn<<endl;
else cout<<-1<<endl;
}
