#include <stdio.h>

int a[9001];
int aparitii[7001];
int numarPerElement[9001];
int N;

int main()
{
	freopen ("medie.in","r",stdin);  
	freopen ("medie.out","w",stdout);
	scanf("%d",&N);
	
	int val;
	for (int i = 0 ; i < N; i++)
	{
		scanf("%d",&a[i]);
		aparitii[a[i]] ++;
	}

	int sum = 0;
	int tot = 0;
	for (int i = 0; i < N - 1; i++)
	{
		for (int j = i + 1 ; j < N ; j++)
		{
			if (!((a[i]+a[j])&1))
			{
				sum = (a[i] + a[j]) >> 1;
				if (a[i] == sum)
					tot += aparitii[sum] - 2;
				else
					tot += aparitii[sum];
			}
		}
	}

	printf("%d", tot);
	return 0;
}