#include <fstream>
#include <iostream>
using namespace std;
ifstream fin("litere.in");
ofstream fout("litere.out");
int N,mut,i,j,lit[300];
char ch;
int main()
{
    fin>>N;
    for(i=1;i<=N;i++)
    {
        fin>>ch;
        lit[ch]++;
        for(j=ch+1;j<='z';j++)
        {
            mut+=lit[j];
        }
    }
    fout<<mut<<"\n";
    return 0;
}