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

scanf("%d",&n);

for(i=0;i<n;i++)
	{
	scanf("%d",&nr[i]);

	}
for(i=0;i<n;i++)
	{
	for(a=0;a<n-1;a++)
		for(b=a+1;b<n;b++)
			if(a!=i && b!=i)
				{
				l=nr[i];
				u=nr[a];
				k=nr[b];
				if(l==(u+k)/2)
					x++;
				}
	}
printf("%d",x);
return 0;

}