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

#include <stdio.h>

int e[10000];
int ap[10000];
char c[100000];
long long aparitii=0;
int el,n,md;
int main()
{
	int i,j;
	freopen("medie.in","r",stdin);
	freopen("medie.out","w",stdout);
	scanf("%d\n",&n);
	for(i=1;i<=n;i++)
	{
		scanf("%d",&e[i]);
		ap[e[i]]++;
	}
	for(i=1;i<n;++i)
		for(j=i+1;j<=n;++j)
			if((e[i]+e[j])>>1<<1==e[i]+e[j])
			{
				el=0;
				md=(e[i]+e[j])>>1;
				if(e[i]==md)
					++el;
				if(e[j]==md)
					++el;
				if(ap[md]-el>0)
					aparitii=aparitii+ap[md]-el;
			}

	printf("%lld",aparitii);

	return 0;
}