#include <cstdio>
int a[7005],n;
int freq[7005];
int main(){
	freopen("medie.in","r",stdin);
	freopen("medie.out","w",stdout);
	int i,j,k=0,b;
	scanf("%d",&n);
	for(i=0;i<n;++i)
		scanf("%d",&a[i]);
	for(i=0;i<n;++i){
		++freq[a[i]];
	}
	for(i=1;i<n;++i)
		for(j=0;j<i;++j)
			if(i!=j&&(a[i]+a[j])%2==0){
				b=(a[i]+a[j])/2;
				k=k+freq[b];
				if(a[i]==a[j]) k=k-2;
			}
	
	
	
	
	
	
	
	printf("%d",k);
}