#include<stdio.h>
int nr[9000],n,i,a,b,x,y;
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++)
	{
	y=nr[i]*2;
	for(a=0;a<n-1;a++)
		for(b=a+1;b<n;b++)
			if(a!=i && b!=i)
				{
				if(y==nr[b]+nr[a])
					x++;
				}
	}
printf("%d \n",x);
fclose(stdin);
fclose(stdout);
return 0;

}