#include <iostream>
#include <fstream>

using namespace std;
ifstream fin("medie.in");
ofstream fout("medie.out");

int n,s,i,j,p,v[9005],f[7005];
long long rez;

int main() {
	
	fin>>n;
	for(i=1;i<=n;i++)
	{
		fin>>v[i];
		f[v[i]]++;
	}
	
	for(i=1;i<n;i++)
	{
		p=v[i]%2;
		for(j=i+1;j<=n;j++)
			if(v[j]%2==p)
			{
				s=(v[i]+v[j])/2;
				if(s==v[i]||s==v[j])
				{
					f[v[i]]--;
					f[v[j]]--;
					rez+=f[s];
					f[v[i]]++;
					f[v[j]]++;
				}
				else rez+=f[s];
			}
	}
	fout<<rez<<"\n";
}