#include<fstream>
#define dmax 9003
using namespace std;
ifstream in("medie.in");
ofstream out("medie.out");
int x[dmax],sol,n,t[dmax];
int main()
{	int i,j;
	in>>n;
	for(i=0;i<n;i++)
	{	in>>x[i];
		t[x[i]]++;
	}	
	in.close();
	for(i=0;i<n;i++)
		for(j=i+1;j<n;j++)
		{	if(x[i]!=x[j] && (x[i]+x[j])%2==0 )
				sol+=t[(x[i]+x[j])/2];
			else if(x[i]==x[j])
				if(t[x[i]]>2)
					sol+=(t[x[i]]-2);
			}
	out<<sol;
	out.close();
	return 0;
}