#include<stdio.h>
#define NMAX 1<<14
#define MMAX 1<<13
int n,i,j,a[NMAX],v[MMAX],b[MMAX];
long t=0;
int main(){
	freopen("medie.in","r",stdin);
	freopen("medie.out","w",stdout);
	scanf("%d",&n);
	for(i=1;i<=n;i++)
		scanf("%d",&a[i]);
	for(i=1;i<n;i++){
		v[a[i]]++;
		for(j=i+1;j<=n;j++)
			if((a[i]+a[j])%2==0)
				b[(a[i]+a[j])>>1]++;
	}
	v[a[n]]++;
	for(i=1;i<=n;i++)
		if(b[a[i]])
			t=t+b[a[i]]-v[a[i]]+1;
	printf("%ld\n",t);
	fclose(stdin);
	fclose(stdout);
	return 0;   
}