#include <fstream>
using namespace std;
int n,i,j,sol,f[200];
char v[10010];
int main () {
    ifstream fin ("litere.in");
    ofstream fout("litere.out");
    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;
}