#include <stdio.h>  
int i,j,n,nrtot,max;  
int nr[7010],fol[14010];  
int main()  
{  
  int a[9010];
  freopen("medie.in","r",stdin);  
  freopen("medie.out","w",stdout);  
      
  scanf("%d",&n);  
  for (i=1; i<=n; i++)  
  {  
     scanf("%d",&a[i]);  
     if (a[i]>max) max=a[i];  
          nr[a[i]]++;  
   }  
   for (i=1; i<=n-1; i++)  
       for (j=i+1; j<=n; j++)  
       if ((a[i]+a[j])%2==0)  
          fol[(a[i]+a[j])/2]++;                       
   
   nrtot=0;  
   for (i=1; i<=max; i++)  
     if (fol[i]!=0 && nr[i]!=0) nrtot+=nr[i]*fol[i]-nr[i]*(nr[i]-1);         
               
       
   printf("%d\n",nrtot);  
         
         
   return 0;      
}