#include<iostream>
#include<stdio.h>
#include<vector>
using namespace std;
int main()
{
	freopen("medie.in","r",stdin);
	freopen("medie.out","w",stdout);
	int n,i,j;
	int nr=0;
	scanf("%d",&n);
	vector<long> a,v(7010,0);
	a.push_back(0);
	for(i=1;i<=n;i++)
	{
		scanf("%d",&j);
		a.push_back(j);
		v.at(j)++;
	}
	for(i=1;i<n;i++)
		for(j=i+1;j<=n;j++)
			if((a.at(i)+a.at(j))%2==0)
			{
				nr+=v.at((a.at(i)+a.at(j))/2);
				if(a.at(i)==(a.at(i)+a.at(j))/2) nr--;
				if(a.at(j)==(a.at(i)+a.at(j))/2) nr--;
			}
	printf("%d\n",nr);
	return 0;
}