#include<stdio.h>
int nr[9000],c[9000],d[9000],n,i,a,b,x,y;
int main(void)
{
freopen("MEDIE.IN","r",stdin);
freopen("medie.out","w",stdout);

scanf("%d",&n);

for(i=0;i<n;i++)
	{
	scanf("%d",&nr[i]);
	d[i]=nr[i]*2;
	}
for(a=0;a<n-1;a++)
	for(b=a;b<n;b++)
		for(i=0;i<n;i++)
			if(d[i]==nr[a]+nr[b] && i!=a && i!=b)
                        	x++;




printf("%d \n",x);
fclose(stdin);
fclose(stdout);
return 0;

}