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