#include<algorithm>
#include<iostream>
using namespace std;

int main()
{
	int n,i,j,k,count=0, a[100] = {0};
	cin >> n;
	for (i = 0; i < n; i++)
	{
		cin >> a[i];
	}
	sort(a, a + 100);
	int l=0, r = n - 1;
	for (j = 2; j < n; j++) {
		int target = a[j];
		l = 0; r = n - 1;
		while (l < r)
		{
			if (a[l] + a[r] == target)
			{
				count++; r--; l++;
			}
			else if (a[l] + a[r] < target)l++;
			else r--;
		}
	}
	cout << count;
	return 0;
}