description: <image>
  
  William has two numbers a and b initially both equal to zero. William mastered performing three different operations with them quickly. Before performing each operation some positive integer k is picked, which is then used to perform one of the following operations: (note, that for each operation you can choose a new positive integer k)
  
    1. add number k to both a and b, or 
    2. add number k to a and subtract k from b, or 
    3. add number k to b and subtract k from a. 
  
  
  
  Note that after performing operations, numbers a and b may become negative as well.
  
  William wants to find out the minimal number of operations he would have to perform to make a equal to his favorite number c and b equal to his second favorite number d.
  
  Input
  
  Each test contains multiple test cases. The first line contains the number of test cases t (1 ≤ t ≤ 10^4). Description of the test cases follows.
  
  The only line of each test case contains two integers c and d (0 ≤ c, d ≤ 10^9), which are William's favorite numbers and which he wants a and b to be transformed into.
  
  Output
  
  For each test case output a single number, which is the minimal number of operations which William would have to perform to make a equal to c and b equal to d, or -1 if it is impossible to achieve this using the described operations.
  
  Example
  
  Input
  
  
  6
  1 2
  3 5
  5 3
  6 6
  8 0
  0 0
  
  
  Output
  
  
  -1
  2
  2
  1
  2
  0
  
  Note
  
  Let us demonstrate one of the suboptimal ways of getting a pair (3, 5):
  
    * Using an operation of the first type with k=1, the current pair would be equal to (1, 1). 
    * Using an operation of the third type with k=8, the current pair would be equal to (-7, 9). 
    * Using an operation of the second type with k=7, the current pair would be equal to (0, 2). 
    * Using an operation of the first type with k=3, the current pair would be equal to (3, 5). 
solution: # Author : Zahin uddin
  # Github : https://github.com/Zahin52
  
  
  from sys import *
  import math
  #import queue
  input = stdin.readline
  I = int
  R = range
  def listInput(): return list(map(int, input().strip().split()))
  def lineInput(): return map(int, input().strip().split())
  def sJoin(a, sep): return '{}'.format(sep).join(a)
  
  
  def arrJoin(a, sep): return '{}'.format(sep).join(map(str, a))
  # print=stdout.write
  
  
  def isPrime(n):
      if(n <= 1):
          return False
      if(n <= 3):
          return True
      if(n % 2 == 0 or n % 3 == 0):
          return False
      for i in range(5, int(math.sqrt(n) + 1), 6):
          if(n % i == 0 or n % (i + 2) == 0):
              return False
      return True
  
  
  def main():
      for _ in R(I(input())):
          c, d = lineInput()
          add = c+d
          if c == d and c == 0:
              print(0)
          elif c==d :
              print(1)
          elif (add % 2 == 0):
              print(2)
          else:
              print(-1)
  
  
  if __name__ == "__main__":
      main()
  