#include <stdio.h>
#include <assert.h>
#include <string.h>

#define FOR(i,s,d) for(i=(s);i<(d);++i)

int n,sol,A[32];
char s[10111];

int main()
{
	int i,j;
	assert(freopen("litere.in","r",stdin));
	freopen("litere.out","w",stdout);
	assert(scanf("%d",&n)==1);
	assert(n>=1);
	assert(n<=10000);
	assert(scanf("%s",s)==1);
	assert(n==strlen(s));
	FOR(i,0,n)
	{
		sol+=A[s[i]-'a'];
		FOR(j,0,s[i]-'a')
			A[j]++;
	}
	printf("%d\n",sol);
	return 0;
}