#include<fstream>
using namespace std;
ifstream fin("litere.in");
ofstream fout("litere.out");
int v[130],n,k,i,j,sum;
char s[10002];
int main(){
    fin>>n;
    fin>>s;
    sum=0;
    for(i=0;i<n;i++){
        k=0;
        for(j=s[i]+1;j<='z';j++){
            k+=v[j];
        }
        sum+=k;
        v[(int)s[i]]++;
    }
    fout<<sum<<" ";
}