#include <cstdio>
int a[9005];
int freq[7005];
int main(){
	int i,j,rez=0,x,n;
	scanf("%d",&n);
	for(i=1;i<=n;++i)
		scanf("%d",&a[i]);
	for(i=1;i<=n;++i)++freq[a[i]];
	for(i=2;i<=n;++i)
		for(j=1;j<i;++j)
			if((a[i]+a[j])%2==0){
				x=(a[i]+a[j])/2;
				rez+=freq[x];//de cate ori apare aceasta medie in sir
				if(a[i]==a[j])
					rez-=2;
			}
	printf("%d",rez);
}