#include <iostream>
#include <fstream>
#include <algorithm>
using namespace std;
char c;
int k,i,m,ct,x,f[30];
int main()
{
    ifstream fin("litere.in");
    ofstream fout("litere.out");
    fin>>x;
    for(k=0;k<x;k++)
    {
        fin>>c;
        f[c-'a']++;
        for(i=c+1;i<='z';i++)
            ct+=f[i-'a'];

    }
    fout<<ct<<endl;
    return 0;
}