#include<stdio.h>

int a[20000],b[1000],i,j,n,nr=0;

int main()
{ 
freopen("medie.in","r",stdin);
freopen("medie.out","w",stdout);
    scanf("%d",&n);
    
for(i=1;i<=n;i++) {  scanf("%d",&b[i]);
                         a[2*b[i]]++;
                      }
 for(i=1;i<=n;i++)
for(j=i+1;j<=n;j++) if(a[b[i]+b[j]]) {    if(b[i]==(b[i]+b[j])/2)  nr+=(a[b[i]+b[j]]-2);
                                                 else nr+=a[b[i]+b[j]];
                                            }     
       
  printf("%d\n",nr);
    
    return 0;
}