#include <stdio.h>  
int i,j,n,nrtot,max;  
int a[9010],nr[7010],fol[14010];  
  
int main()  
{  

   freopen("medie.in","r",stdin);  
   freopen("medie.out","w",stdout);  

   scanf("%d",&n);
   max=0;  
   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])>>1]++;                       
   
   nrtot=0;  
   for (i=1; i<=max; i++)  
       if (fol[i]!=0 && nr[i]!=0) nrtot+=nr[i]*(fol[i]-nr[i]+1);         

   printf("%d\n",nrtot);  
       
       
   return 0;      
}