#include<stdio.h>
long long int n,i,j,a,c[7001],x[9001],sol;
int main()
{
	FILE *f,*g;
	f=fopen("medie.in","r");
	g=fopen("medie.out","w");
	fscanf(f,"%lld",&n);
	for(i=1;i<=n;i++)
	{fscanf(f,"%lld",&x[i]);c[x[i]]++;}
	for(i=1;i<=n-1;i++)
	 for(j=i+1;j<=n;j++)
	   sol+=c[(x[i]+x[j])/2]*((x[i]+x[j]+1)%2);
	fprintf(g,"%lld\n",sol);
	fcloseall();
	return 0;
}