#include<stdio.h>
#include<string.h>

int main()
{
	int i,j,v[9000],nr[7001],total=0,n,m;
	FILE *f=fopen("medie.in","r");

	memset(nr,0,sizeof(nr));
	fscanf(f,"%i",&n);
	for(i=0;i<n;i++)
	{
		fscanf(f,"%i",&v[i]);
		nr[v[i]]++;
	}
	fclose(f);

	for(i=0;i<n;i++)
	{
		for(j=i+1;j<n;j++)
		{
			if((v[i]+v[j])%2==0 && v[i]!=v[j])
			{
				m=(v[i]+v[j])/2;	
				total+=nr[m];
			}
		}
	}
	f=fopen("medie.out","w");
	fprintf(f,"%i\n",total);
	fclose(f);
	return 0;
}