#include<stdio.h>
#define NMAX 9500
long i,j,n,m,k,l,a,s,x[NMAX],y[NMAX],z[NMAX];
int main()
{
	freopen("medie.in","r",stdin);
	freopen("medie.out","w",stdout);
	scanf("%ld",&n);
	for (i=1;i<=n;i++)
	{
		scanf("%ld",&x[i]);
		z[x[i]]++;
	}

	for (i=1;i<=n;i++)
		for (j=i+1;j<=n;j++)
		{
			a=x[i]+x[j];
			if (a/2*2==a)
			y[a/2]++;
			if (x[i]==x[j]&&y[x[i]]) s-=2;
		}
	for (i=1;i<=7001;i++)
		if (y[i])
			if(z[i])
				s+=y[i]*z[i];
/*	for (i=1;i<=n;i++)
		for (j=i+1;j<=n;j++)
			if (x[i]==x[j]&&y[x[i]])
				s-=2;               */

	printf("%ld\n",s);


	return 0;
}