Doubles

You are given a list of 2 to 15 distinct natural numbers. Count how many numbers in the list have their exact double (the value $2x$) also present in the same list.

For example, if the list is 1 4 3 2 9 7 18 22, then 2 is the double of 1, 4 is the double of 2, and 18 is the double of 9, so the answer is 3.

The input consists of several test cases. Each test case is given on a single line: 2 to 15 distinct natural numbers separated by spaces, followed by a 0. The 0 is not part of the list and marks the end of that list. Each natural number is less than 100. After the last test case, a single line containing -1 marks the end of the input.

For each test case, print on its own line the number of values whose exact double is also present in the list.