#include <stdio.h>

long i, j, n, num, medii[7000], x[9000], frecv[7000];

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]), ++frecv[x[i]];

	for (i = 0; i < n - 1; ++i)
		for (j = i + 1; j < n; ++j)
		{
			if ((x[i] + x[j]) % 2 == 0)
				++medii[(x[i] + x[j]) / 2];
		}

	for (i = 0; i < n; ++i)
		for (j = 0; j <= 7000; ++j)
		{
			if (x[i] == j)
				num += medii[j] - frecv[x[i]] + 1;
		}
	printf("%ld\n", num);
	return 0;
}