#include<cstdio>
#include<algorithm>
using namespace std;
int x,nrt,n,a[10001],f[10001];
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]);
		f[a[i]]++;
	}
	sort(a+1,a+n+1);
	for(int i=1;i<n;i++)
		for(int j=i+1;j<=n;j++)
		{
			x=a[i]+a[j];
			if(x%2==0)
			{
				x/=2;
				nrt+=f[x];
				if(a[j]==x)
					nrt-=2;
			}
		}
	printf("%d",nrt);
	return 0;
}