#include<stdio.h>
int v[7010];
int main(){
	freopen("medie.in","r",stdin);
	freopen("medie.out","w",stdout);
	int n,i,aux,max=0,j;
	long long s=0,s1;
	
	scanf("%d",&n);
	for(i=0;i<n;i++){
		scanf("%d",&aux);
		v[aux]++;
		if(aux>max)
			max=aux;
	}
	
	for(i=1;i<=max;i++){
		if(v[i]){
			if(v[i]>2){
				s1=(v[i]-1)*(v[i]-2);
				s1/=2;
				s+=v[i]*s1;
			}
			for(j=1;j<i;j++){
				if(v[j]){
					aux=2*i-j;
					if(aux<=7000)
						s+=v[j]*v[aux];
				}
			}
		}
	}
	
	printf("%lld\n",s);
	
	return 0;
}