#include<fstream.h>
#define NMAX 14001
#define NMAX2 9002

int fr[NMAX], i, n, a[NMAX2], j, s, nr=0;

int main()
{
	ifstream f("medie.in");
	ofstream g("medie.out");
	
	f>>n;
	
	for(i=1; i<=n; ++i)
	{
		f>>a[i];
		++fr[a[i]];
	}
	
	for (i=2; i<=n; ++i)
		for (j=1; j<i; ++j)
		{
			s=a[i]+a[j];
			if (s % 2==0) 
				{
					nr+=fr[s>>1];
					if (a[i]==a[j])nr-=2;
				}
		}
		
	g<<nr<<"\n";
	
	f.close();
	g.close();
	return 0;
}