#include<stdio.h>
#include<string.h>
#define Nmx 9004
#define Mmx 7004

int a[Nmx];
int uz[Mmx],ver[Mmx],n;

void solve()
{
	for(int i=1;i<=n;++i)
	{
		for(int j=i+1;j<=n;++j)
		{
			if((a[i]+a[j])%2==0)
				++ver[(a[i]+a[j])>>1];
		}
	}
}

void afis()
{	
	int nr=0;
	for(int i=1;i<=n;++i)
		if(ver[a[i]])
			nr+=(ver[a[i]]-uz[a[i]]+1);
	printf("%d\n",nr);
}

int main()
{
	freopen("medie.in","r",stdin);
	freopen("medie.out","w",stdout);
	scanf("%d",&n);
	for(int i=1;i<=n;++i)
		{scanf("%d",&a[i]);
	     ++uz[a[i]];
		}
	solve();
	afis();
	return 0;
}