#include <fstream>
using namespace std;
ifstream fin("litere.in");
ofstream fout("litere.out");
int i,j,ap,a[200],n;
char v[10001];
int main(){
    fin>>n;
    for(i=1;i<=n;i++)
        fin>>v[i];
    for(i=1;i<=n;i++){
        for(j=v[i]+1;j<='z';j++)
            ap+=a[j];
        a[v[i]]++;
    }
    fout<<ap;
    return 0;
}