#include <stdio.h>

#define nmax 9005
#define vmax 7005

int tot[vmax * 2],cate[vmax],v[nmax],i,j,n,rez = 0;

int main() {
	freopen("medie.in","r",stdin);
	freopen("medie.out","w",stdout);

	scanf("%d",&n);
	for(int i = 1; i <= n; i++) {
		scanf("%d",&v[i]); cate[v[i]]++;
		for(int j = 1; j < i; j++) tot[v[i] + v[j]]++;
	}

	for(int i = 1; i <= n; i++) rez += (tot[v[i] + v[i]] - cate[v[i]] + 1);

	printf("%d\n",rez);

	return 0;
}