#include <cstdio>
using namespace std;

FILE* fin = fopen ("medie.in", "r");
FILE* fout = fopen ("medie.out", "w");

#define MAXN 9010
#define MAXV 7010

int f[MAXV], m[MAXV], v[MAXN], n;

int main ()
{
	fscanf (fin, "%d\n", &n);
	
	for (int i = 0; i < n; ++i) {
		fscanf (fin, "%d\n", &v[i]);
		f[v[i]] ++;	
		for (int j = 0; j < i; ++j) {
			if (!((v[i] + v[j])&1)) {
				m[(v[i] + v[j]) >> 1] ++;
			}
		}
	}
	
	int rez = 0;
	for (int i = 0; i < n; ++i) {
		if (m[v[i]]) {
			rez += m[v[i]] - f[v[i]] + 1;			
		}	
	}
		
	fprintf (fout, "%d\n", rez);
	
	fclose (fin);
	fclose (fout);
	return 0;
}