// litere.cpp : Defines the entry point for the console application.
//

//#include "stdafx.h"
#include "fstream"
using namespace std;
int main()
{
	ifstream f("litere.in");
	ofstream g("litere.out");
	int n,i,j;
	char v[10001];
	int a[27];
	f>>n>>v;
	for(i=0;i<26;i++)
		a[i]=0;
	long long s=0;
	for(i=0;i<=n-1;i++)
	{
		for(j=v[i]-97+1;j<='z'-97;j++)
			s=s+a[j];
		a[v[i]-97]++
	}
	g<<s;
	return 0;
}