#include<cstdio>
#include<iostream>
#include<algorithm>
#include<vector>
#include<queue>
#include<cstring>
#include<set>
#include<map>
#include<cmath>
using namespace std ;

#define maxlit 30

int N ;

int cate[maxlit] ;

int sol ;

int main()
{
	std::ios_base::sync_with_stdio(false) ;

	freopen("litere.in", "r", stdin);
	freopen("litere.out", "w", stdout);

    scanf("%d\n", &N);

    char ch ;

    for(int i = 0; i < N; ++i)
    {
        scanf("%c", &ch);

        ++cate[ ch - 'a' ] ;

        for(int j = ch + 1 - 'a'; j <= 'z' - 'a'; ++j )
            sol += cate[j] ;
    }

    printf("%d", sol);

	return 0 ;
}