#include <bits/stdc++.h>
using namespace std;

ifstream fin("litere.in");
ofstream fout("litere.out");

int fv[27];

int main(){
    
    int n,schimb=0;
    
    fin >> n;
    
    char c;
    for(int i = 0;i < n;i++){
        fin >> c;
        
        int x = c-'a';
        
        for(int j = x+1;j <= 26;j++){
            schimb += fv[j];
        }
        
        fv[x]++;
    }
    
    fout << schimb;
    
    
    return 0;
}