#include <fstream>
using namespace std;
ifstream cin("litere.in");
ofstream cout("litere.out");
int n, i, j, f[150];
long long nr;
char x;
int main() {
    cin>>n;
    for(i=1;i<=n;i++){
        cin>>x;
        for(j=x+1;j<='z';j++)
            nr+=f[j];
        f[x]++;
    }
    cout<<nr;


}