/**
* file homwork.cpp
* author ??? 1100012722
* data 2011-10-20
* description ??
*/ 
int main()
{
 int a[20] , b[20] ;
 int i , j , k ;
 int n ;
 i = 1 ; n = 0 ;
 
 while( 1 )             // ??????
  {
   scanf("%d",&a[i]);
   if( a[i] == -1 )  break;     // ??-1???????
   else if( a[i] == 0 )         // ??0???????
     {
       for(j = 1 ; j <= i-1 ; j++)
	  b[j] = a[j] * 2;

       for(j = 1 ; j <= i-1 ; j++)
	  for(k = 1 ; k <= i-1 ; k++)
	    {
	     if( b[j] == a[k] ) n++;
	    }
		
       printf("%d\n",n);
       i = 1;
       n = 0;                // ?????????
     }
   else i ++;                // ?0?-1????
  } 
  return 0;
}