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

using namespace std;
ifstream fin ("litere.in");
ofstream fout("litere.out");
int n,i,j,f[10000],sol;
char c;

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