#include<iostream>
#include<cstdio>
using namespace std;
long long a, b[1000001], ans = 0,c[10000001],num=1;
int main()
{
	cin >> a;
	for (int i = 1; i <= a; i++)
	{
		cin >> b[i];
	}
	for (int i = 1; i <= a; i++)
	{
		for (int j = a; j >= 1 + i; j--)
		{
			c[num] = b[i] + b[j];
			num++;
		}
	}
	
	for (int i = 1; i <= a; i++)
	{
		for (int j = 1; j <= a * (a - 1) / 2; j++)
		{
			if (b[i] == c[j])
			{
				ans++;
			}
		}
	}
	cout << ans;
	
	
}