#include<stdio.h>
#define NMAX 1<<14;
#define MMAX 1<<13;
FILE *f,*g;
int n,i,j,a[NMAX],frecv[MMAX],b[MMAX];
long total=0;
int main()
{
	f=fopen("medie.in","r");
	g=fopen("medie.out","w");
	fscanf(f,"%d",&n);
	for(i=1;i<=n;i++)
		fscanf(f,"%d",&a[i]);
	for(i=1;i<n;i++)
	{
		frecv[a[i]]++;
		for(j=i+1;j<=n;j++)
			if((a[i]+a[j])%2==0)
		b[(a[i]+a[j])/2]++;
	}
frecv[a[n]]++;
for(i=1;i<=n;i++)
	if(b[a[i]])	
		total=total+b[a[i]]-frecv[a[i]]+1;
fprintf(g,"%ld",total);
fclose(f);
fclose(g);
return 0;
}