#include <iostream>
#include <fstream>
#include <string>

using namespace std;

int main(){
    ifstream f("litere.in");
    ofstream g("litere.out");

    int n;
    f>>n;
    string s;
    f>>s;
    int fr[26]={0};
    int inv=0;

    for (int i=n-1;i>=0;i--) {
        int x=s[i]-'a';
        int mmd=0;
        for (int e=0;e<x;c++) mmd+=fr[e];
        inv+=mmd;
        fr[x]++;
    }
    
    g<<inv;

    f.close();
    g.close();
    return 0;
}