#include<cstdio>
#include<algorithm>

using namespace std;

const int N=1<<14;

short int a[N],fr[N],b[N];

int main()
{
	freopen("medie.in","r",stdin);
	freopen("medie.out","w",stdout);
	int n;
	scanf("%d",&n);
	for (int i=1;i<=n;i++)
	{
		scanf("%hd",&a[i]);
		fr[a[i]]++;
	}
	n=0;
	for (int i=1;i<=7000;i++)
		for (int j=1;j<=fr[i];j++)
		{
			n++;
			b[n]=i;
		}
	int s=0;
	for (int j=1;j<n;j++)
		for (int k=j+1;k<=n;k++)
		{
			int x=b[j]+b[k];
			if (x%2==0)
			{
				x/=2;
				s+=fr[x];
				if (b[j]==x)
					s--;
				if (b[k]==x)
					s--;
			}
		}
	printf("%d",s);
	return 0;
}