#include <stdio.h>

long i, j, n, num, medii[15000], x[10000], frecv[8000];

int main()
{
	freopen ("medie.in", "rt", stdin);
	freopen ("medie.out", "wt", stdout);

	scanf("%ld", &n);
	for (i = 0; i < n; ++i)
		scanf("%ld", &x[i]);

	for (i = 0; i < n - 1; ++i)
	{
		++frecv[x[i]];
		for (j = i + 1; j < n; ++j)
			++medii[(x[i] + x[j])];
	}
	++frecv[x[n - 1]];

	for (i = 0; i < n; ++i)
		if (medii[x[i] * 2])
			num += medii[x[i] * 2] - frecv[x[i]] + 1;


/*	for (i = 0; i < n - 1; ++i)
		for (j = i + 1; j < n; ++j)
		{
			++medii[(x[i] + x[j])];
		}

	for (i = 0; i < n; ++i)
		for (j = 0; j <= 14000; j += 2)
		{
			if (x[i] == (j >> 1))
				num += medii[j >> 1] - frecv[x[i]] + 1;
		}*/
	printf("%ld\n", num);
	return 0;
}