#include <iostream>
#include <fstream>
using namespace std;
int main(){
    ifstream f("litere.in");
    ofstream g("litere.out");
    int n,fr[26]={0},cnt=0;
    char x;
    f>>n;
    for(int i=0;i<n;i++){
        f>>x;
        int p=x-'a';
        for(int j=p+1;j<26;j++)
            cnt+=fr[j];
        fr[p]++;
    }
    g<<cnt;
    return 0;
}