#include <stdio.h>
#include <stdlib.h>
#include <string.h>
# define MaxN 9002
# define MaxX 7002
# define div2(x) ( ! ((x) & 1) )
int  a[MaxN], rep[MaxX], n;
int  sol;
int v[MaxX];
int main()
{
	int i;
	freopen ("medie.in", "r", stdin);
	freopen ("medie.out","w",stdout);
	scanf("%d", &n);
	int x;
	for( i=1; i<=n; i++) {scanf("%d", &x); a[i]=x;}
	int  j;
	for(i=1; i<n; i++)
	{
		++rep[ a[i] ];
		for(j=i+1; j<=n; j++)
			if ( (a[i]+a[j] )%2==0 ) ++v[(a[i]+a[j])/2];
	}
	++rep[a[n]];
	for(i=1; i<=n; i++)
		if ( v[ a[i] ] ) sol += long( v[ a[i] ] - rep[ a[i] ] + 1 );
	printf("%d\n", sol);
	return 0;
}