#include <stdio.h>
#define Nmax 9003
#define Vmax 7003

int v[Nmax], fv[Vmax], M[Vmax];
int n, i, j, m;
long long sol;

int main() {
	
	FILE *f = fopen("medie.in", "r");
	FILE *g = fopen("medie.out", "w");
	
	fscanf(f, "%d", &n);
	
	for (i = 1; i <= n; i++) {
		fscanf(f, "%d", &v[i]);
		fv[v[i]]++;
	}
	
	for (i = 1; i < n; i++)
		for (j = i+1; j <= n; j++) {
			m = v[i] + v[j];
			if ((m & 1) == 0)
				M[m>>1]++;
		}
	
	for (i = 1; i <= n; i++)
		sol += M[v[i]] - (fv[v[i]] - 1);
	
	fprintf(g, "%lld", sol);
	
	fclose(f);
	fclose(g);
	
	return 0;
}