#include <bits/stdc++.h>

using namespace std;

ifstream fin("litere.in");
ofstream fout("litere.out");

int fr[130],strnum,x;
char c;

int main()
{
    fin >> strnum;
    for(int i=1; i<=strnum; ++i){
        fin >> c;
        fr[c]++;
        for(int j=c+1;j<='z'; ++j){
            x+=fr[j];
        }
    }
    fout << x;

    return 0;
}