#include<stdio.h>
int main()
{
    int math[100001],i,j,n,count=0;
    bool r[100001]
    scanf("%d",&n)
    for(i=0;i<n;i++)
    {
        scanf("%d",&math[i]);
        r[math[i]]=true;
        }
         for(i=0;i<n-1;i++) 
         {for(j=i+1;j<n;j++)  
             {if(r[math[i]+math[j]])
                 {count++;count+1;
                 
                r[math[i]+math[j]]=false;} 
                }
                }printf("%d",count);
                return 0;
}