// medie.cpp : Defines the entry point for the console application.
//

#include <stdio.h>

int e[10000];
int ap[10000];
int aparitii=0;
int el,n,md;

int main()
{
	freopen("medie.in","r",stdin);
	freopen("medie.out","w",stdout);
	scanf("%d",&n);
	for(int i=1;i<=n;i++)
	{
		scanf("%d",&e[i]);
		ap[e[i]]++;
	}
	for(int i=1;i<=n;i++)
		for(int j=i+1;j<=n;j++)
			if(((float)(e[i]+e[j])/2)==(e[i]+e[j])/2)
			{
				el=0;
				md=(e[i]+e[j])/2;
				if(e[i]==md)
					el++;
				if(e[j]==md)
					el++;
				if(ap[md]-el>0)
					aparitii=aparitii+ap[md]-el;
			}

	printf("%d",aparitii);

	return 0;
}