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: import queue
  from math import *
  import sys
  from collections import *
  from random import *
  
  sys.setrecursionlimit(99999)
  eps = sys.float_info.epsilon
  P = 2
  INF = 1e9 + 1
  MOD = 1000000007
  
  
  def is_prime(n):
      if n == 0 or n == 1:
          return False
      d = 2
      while d * d <= n:
          if n % d == 0:
              return False
          d += 1
      return True
  
  
  def div_up(n, k):
      if n % k == 0:
          return n // k
      else:
          return n // k + 1
  
  
  def num_len(n, base):
      if n == 0:
          return 1
      return int(floor(log(n, base) + 1))
  
  
  def dfs_sample(graph, cl, p, v):
      cl[v] = 1
      for to in graph[v]:
          if cl[to] == 1 and p[v] != to:
              # yay, cycle
              pass
          elif cl[to] == 0:
              p[to] = v
              dfs_sample(graph, cl, p, to)
      cl[v] = 2
  
  
  def down(a):
      for i in range(1, len(a)):
          if a[i] > a[i - 1]:
              return False
      return True
  
  
  def up(a):
      for i in range(1, len(a)):
          if a[i] < a[i - 1]:
              return False
      return True
  
  
  def code(c):
      return ord(c) - 32
  
  
  def _hash_(s):
      res, p = 0, 1
      for i in range(len(s)):
          res += (code(s[i]) * p)
          res %= MOD
          p *= P
          p %= MOD
      return res % MOD
  
  
  def remove_edge(v, u, graph):
      graph[v].remove(u)
      graph[u].remove(v)
  
  
  def dfs_stack(start, graph, visited):
      global b
      stack = []
      visited[start] = True
      stack.append(start)
      while len(stack) != 0:
          v = stack.pop()
          for to in graph[v]:
              if not visited[to]:
                  visited[to] = True
                  stack.append(to)
  
  
  def solve():
      c, d = map(int, input().split())
      if c == d == 0:
          print(0)
          return
      if abs(c - d) % 2 != 0:
          print(-1)
      else:
          if c == d:
              print(1)
          else:
              print(2)
  
  
  
  for _ in range(int(input())):
      solve()
  
  
  def debug():
      pass
  
  
  # debug()