#include <iostream>
#include <fstream>
using namespace std;
int n,ct,v[33];
string s;
int main()
{
    ifstream f ("litere.in");
    ofstream g ("litere.out");
    f>>n>>s;
    n--;
    for (int i=0;i<=n;i++)
    {
        int x=s[i]-'a';
        v[x]++;
        for (int j=x+1;j<='z'-'a';j++)
            ct+=v[j];
    }
    g<<ct;
}