/// Litere -- varianta eficienta
#include <iostream>
#include <fstream>

using namespace std;

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

int fr[10005];
char s;

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