#include <fstream>
using namespace std;
ifstream cin("char.in");
ofstream cout("char.out");
char x;
int f[130], i, j, n, nr;
int main (){
    cin>>n;
    for(i=1;i<=n;i++){
        cin>>x;
        for (j=x+1;j<='z';j++)
            nr=nr+f[j];
        f[x]++;
      }
      cout<<nr;
}