#include <stdio.h>

FILE *f,*s;

int m,n,i,j,v1[10000],v2[10000];
long long int rez;

int main()
{
	f=fopen("medie.in","r");
	s=fopen("medie.out","w");
	
	fscanf(f,"%d\n",&n);
	
	for(i=1;i<=n;i++)
		fscanf(f,"%d\n",&v1[i]);
	
	for(i=1;i<n;i++)
	{
		for(j=i+1;j<=n;j++)
		{
			m=v1[i]+v1[j];
			
			v2[m]++;
		}	
	}	
	
	for(i=1;i<=n;i++)
	{
		rez+=v2[v1[i]*2];
	}	
	
	fprintf(s,"%lld",rez);
	
	fclose(s);
	
	return 0;
}