#include<fstream>
#include<algorithm>
using namespace std;
ifstream fin("litere.in");
ofstream fout("litere.out");
int n,i,s,j;
char v[10001],maxim;
int f[124];
int main(){
    fin>>n;
    fin>>v;
    for(i=0;i<n;i++){
        f[v[i]]++;
        //s = 0;
        for(j=v[i]+1;j<='z';j++){
            s+=f[j];
        }
    }

    fout<<s;
    return 0;
}