#include<stdio.h>
int nr[15000];
int main(){
	freopen("medie.in","r",stdin);
	freopen("medie.out","w",stdout);
	int n,i,j,s=0,max,aux,i1;
	
	scanf("%d",&n);
	max=0;
	for(i=0;i<n;i++){
		scanf("%d",&aux);
		nr[aux]++;
		if(aux>max)
			max=aux;
	}
	
	s=0;
	for(i=0;i<=max;i++){
		if(nr[i]){
			i1=2*i;
			for(j=1;j<i;j++)
				s+=nr[j]*nr[i1-j];
			if(nr[i]>=3)
				s+=nr[i]*(((nr[i]-1)*(nr[i]-2))/2);
		}
	}
	
	printf("%d\n",s);
	
	return 0;
}