#include <fstream>
#include <algorithm>
#include <cmath>
#define DIM 10002

using namespace std;

ifstream f("litere.in");
ofstream g("litere.out");

int n, s, frecv[DIM];

char sir[DIM];

pair<int, int> v[DIM];

int main()
{
    f>>n;
    f>>sir + 1;


    for(int i = n; i >= 1; -- i){
        s += frecv[sir[i]];
        for(int j = sir[i] + 1; j <= 'z'; ++ j)
            ++ frecv[j];
    }



    g<<s;

    return 0;
}