#include<stdio.h>
long long int n,i,j,a,c[7001],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",&a);c[a]++;}
	for(i=1;i<=6998;i++)
	if(c[i])
	{ sol+=c[i]*(c[i]-1)*(c[i]-2);
	 for(j=i+1;2*j<=i+7000;j++)
	  sol+=c[i]*c[j]*c[2*j-i];
	}
	sol/=2;
	fprintf(g,"%lld\n",sol);
	fcloseall();
	return 0;
}