#include <iostream>
#include <fstream>
#include <algorithm>

using namespace std;

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

const int NMAX=10005;
int frecv[NMAX];
char s;

int main()
{
    int n,total=0,i,j;
    fin>>n;
    for(i=1;i<=n;i++)
    {
        fin>>s;
        frecv[s]++;
        for(j=s+1;j<='z';j++)
        {
            total=total+frecv[j];
        }
    }
    fout<<total;
    return 0;
}