#include<iostream.h>
#include<stdio.h>
 int main()
{
 freopen("medie.in","r",stdin);
 freopen("medie.out","w",stdout);
  long n,i,j,a[10000],d=0,nr=0,v[50000];
   scanf("%ld\n",&n);
 for(i=1;i<=n;i++)
 scanf("%ld\n",&a[i]);
  for(i=1;i<=n;i++)
   for(j=i+1;j<=n;j++)
     if((a[i]+a[j])%2==0) { d++; v[d]=(a[i]+a[j])/2; }
 for(i=1;i<=n;i++)
  for(j=1;j<=d;j++)
   if(a[i]==v[j]) nr++;
printf("%ld\n",nr);
 return 0;
}