#include <iostream>
#include <fstream>

using namespace std;

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

int n, sol;
int fr[305];

int main()
{
    fin >> n;
    char ch;
    for(int i = 1; i <= n; i++)
    {
        fin >> ch;
        fr[ch]++;
        for(int j = ch + 1; j <= 'z'; j++)
        {
            sol += fr[j];
        }
    }
    fout << sol;
    return 0;
}