#include<cstdio>
#include<algorithm>
using namespace std;
const int Q=1<<14;
short int x,n,a[Q],f[Q];
int nrt;
int main()
{
	freopen("medie.in","r",stdin);
	freopen("medie.out","w",stdout);
	scanf("%hd",&n);
	for(int i=1;i<=n;++i)
	{
		scanf("%hd",&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+=(int)f[x];
				if(a[j]==x)
					nrt-=2;
			}
		}
	printf("%d",nrt);
	return 0;
}