#include <fstream>
using namespace std;
ifstream fin ("litere.in");
ofstream fout("litere.out");
char v[10010];
int f[130];
int sol, n, i, j;
int main () {
    fin>>n;
    for (i=1;i<=n;i++)
        fin>>v[i];
    for (i=1;i<=n;i++){
        for (j=v[i]+1;j<='z';j++)
            sol+=f[j];
        f[v[i]]++;
    }
    fout<<sol;
    return 0;
}