#include <iostream>
#include <fstream>


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=x+fr[j];
        }
    }
    fout<<x;

    return 0;
}