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: # SHRi GANESHA    author: Kunal Verma #
  
  import os
  import sys
  from collections import Counter
  
  from itertools import *
  
  from io import BytesIO, IOBase
  
  
  def lcm(a, b):
      return (a * b) // gcd(a, b)
  
  
  '''
      mod = 10 ** 9 + 7
      fac = [1]
      for i in range(1, 2 * 10 ** 5 + 1):
          fac.append((fac[-1] * i) % mod)
      fac_in = [pow(fac[-1], mod - 2, mod)]
      for i in range(2 * 10 ** 5, 0, -1):
          fac_in.append((fac_in[-1] * i) % mod)
      fac_in.reverse()
      def comb(a, b):
          if a < b:
              return 0
          return (fac[a] * fac_in[b] * fac_in[a - b]) % mod
  '''
  
  
  # MAXN = 10000004
  # spf = [0 for i in range(MAXN)]
  # adj = [[] for i in range(MAXN)]
  def sieve():
      global spf, adj, MAXN
      spf[1] = 1
      for i in range(2, MAXN):
          spf[i] = i
  
      for i in range(2, MAXN):
          if i * i > MAXN:
              break
          if (spf[i] == i):
              for j in range(i * i, MAXN, i):
                  if (spf[j] == j):
                      spf[j] = i
  
  
  def getdistinctFactorization(n):
      global adj, spf, MAXN
      for i in range(1, n + 1):
          index = 1
          x = i
          if (x != 1):
              adj[i].append(spf[x])
          x = x // spf[x]
          while (x != 1):
              if (adj[i][index - 1] != spf[x]):
                  adj[i].append(spf[x])
                  index += 1
              x = x // spf[x]
  
  
  def printDivisors(n):
      i = 2
      z = [1, n]
      while i <= sqrt(n):
          if (n % i == 0):
              if (n / i == i):
                  z.append(i)
              else:
                  z.append(i)
                  z.append(n // i)
          i = i + 1
      return z
  
  
  def create(n, x, f):
      pq = len(bin(n)[2:])
      if f == 0:
          tt = min
      else:
          tt = max
      dp = [[inf] * n for _ in range(pq)]
      dp[0] = x
      for i in range(1, pq):
          for j in range(n - (1 << i) + 1):
              dp[i][j] = tt(dp[i - 1][j], dp[i - 1][j + (1 << (i - 1))])
      return dp
  
  
  def enquiry(l, r, dp, f):
      if l > r:
          return inf if not f else -inf
      if f == 1:
          tt = max
      else:
          tt = min
      pq1 = len(bin(r - l + 1)[2:]) - 1
      return tt(dp[pq1][l], dp[pq1][r - (1 << pq1) + 1])
  
  
  def SieveOfEratosthenes(n):
      prime = [True for i in range(n + 1)]
      p = 2
      while (p * p <= n):
          if (prime[p] == True):
              for i in range(p * p, n + 1, p):
                  prime[i] = False
          p += 1
      x = []
      for i in range(2, n + 1):
          if prime[i]:
              x.append(i)
      return x
  
  
  def main():
      for  _ in range(int(input())):
          x,y=map(int,input().split())
          if x==y==0:
              print(0)
          elif x==y:
              print(1)
  
          elif (x+y)%2==0:
              print(2)
          else:
              print(-1)
  # Fast IO Region
  BUFSIZE = 8192
  
  
  class FastIO(IOBase):
      newlines = 0
  
      def __init__(self, file):
          self._fd = file.fileno()
          self.buffer = BytesIO()
          self.writable = "x" in file.mode or "r" not in file.mode
          self.write = self.buffer.write if self.writable else None
  
      def read(self):
          while True:
              b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
              if not b:
                  break
              ptr = self.buffer.tell()
              self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
          self.newlines = 0
          return self.buffer.read()
  
      def readline(self):
          while self.newlines == 0:
              b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
              self.newlines = b.count(b"\n") + (not b)
              ptr = self.buffer.tell()
              self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
          self.newlines -= 1
          return self.buffer.readline()
  
      def flush(self):
          if self.writable:
              os.write(self._fd, self.buffer.getvalue())
              self.buffer.truncate(0), self.buffer.seek(0)
  
  
  class IOWrapper(IOBase):
      def __init__(self, file):
          self.buffer = FastIO(file)
          self.flush = self.buffer.flush
          self.writable = self.buffer.writable
          self.write = lambda s: self.buffer.write(s.encode("ascii"))
          self.read = lambda: self.buffer.read().decode("ascii")
          self.readline = lambda: self.buffer.readline().decode("ascii")
  
  
  sys.stdin, sys.stdout = IOWrapper(sys.stdin), IOWrapper(sys.stdout)
  input = lambda: sys.stdin.readline().rstrip("\r\n")
  
  if __name__ == '__main__':
      main()