//
//  main.cpp
//  litere
//
//  Created by she on 24/03/2020.
//  Copyright © 2020 Eusebiu Rares. All rights reserved.
//

#include <iostream>
#include "fstream"

std::fstream in("litere.in", std::ios::in) ;
std::fstream out("litere.out", std::ios::out) ;

int fr[30] ;

int main(int argc, const char * argv[]) {
	int n ; in >> n ;
	int ans(0) ; char curr ;
	for (int i = 1 ; i <= n ; ++ i) {
		in >> curr ;
		for (char j = curr + 1 ; j <= 'z' ; ++ j) {
			ans += fr[j - '0'] ;
		}
		fr[curr - '0'] ++ ;
	}
	out << ans ;
}