#include <cstdio>

int freq[7001],v[7001];

int main()
{
	int n, a[9001];

	FILE *f = fopen("medie.in", "r");
	FILE *g = fopen("medie.out", "w");

	fscanf(f,"%d\n", &n);

	for (int i = 1 ;i <= n ; ++i)
		fscanf(f, "%d\n", &a[i]);

	for (int i = 1 ; i < n ; ++i)
	{
		freq[a[i]]++;
		for (int j = i + 1 ; j <= n ; ++j)
		{
			
			if (!((a[i] + a[j])&1))
				v[((a[i] + a[j]) >> 1)]++;
		}
	}
	freq[a[n]]++;

	unsigned int total = 0;
	for (int i = 1 ; i <= n ; ++i)
	{
		if (v[a[i]])
			total += v[a[i]] - freq[a[i]] + 1;
	}

	fprintf(g, "%d\n", total);

	return 0;
}