#include <iostream>
#include <fstream>
#include <stdio.h>

using namespace std;

ifstream f("litere.in");
ofstream g("litere.out");

char c;
int lit,j,i,n,sol,ap[28];
int main()
{
    f>>n;
    for(i=1;i<=n;i++)
    {
        f>>c;
        lit=c-'a'+1;
        for(j=lit+1;j<=26;j++)
            sol+=ap[j];
        ap[lit]++;
    }


    g<<sol;

    return 0;
}