#include <stdio.h>
#define fin  "medie.in"
#define fout "medie.out"
#define Nmax 9001
#define Cmax 14001

int N,v[Nmax],sum[Cmax],ret;

int main() {
int i,j,tmp;

	freopen(fin,"r",stdin); freopen(fout,"w",stdout);

	scanf("%d",&N);

	for (i=1;i<=N;++i) {
		scanf("%d",&tmp);
		v[i]=tmp;
	}

	for (i=1;i<=N;++i)
	for (j=i+1;j<=N;++j) {
		tmp=v[i]+v[j];
		if ( 2*v[i] == tmp )
			--ret;
		sum[tmp]++;
	}

	for (i=1;i<=N;++i) {
		ret+=sum[2*v[i]];
		sum[2*v[i]]--;
	}

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

	return 0;
}