output_description: Print X.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      N
      x_1 y_1
      :
      x_N y_N
attempt: X = list()
  Y = list()
  N = int(input())
  for i in range(N):
      x, y = map(int, input().split())
      X.append(x)
      Y.append(y)
  print(abs(sum(X)) - abs(sum(Y)))
  
problem_description: Statement
  
  There is an infinitely large triangular grid, as shown below. Each point with
  integer coordinates contains a lamp.
  
  ![](https://img.atcoder.jp/wtf19/f617c94527a62ed72fe7db12b6d1f6b0.png)
  
  Initially, only the lamp at (X, 0) was on, and all other lamps were off. Then,
  Snuke performed the following operation zero or more times:
  
    * Choose two integers x and y. Toggle (on to off, off to on) the following three lamps: (x, y), (x, y+1), (x+1, y).
  
  After the operations, N lamps (x_1, y_1), \cdots, (x_N, y_N) are on, and all
  other lamps are off. Find X.
samples: [{"input": "4\n    -2 1\n    -2 2\n    0 1\n    1 0", "output": "-1\n    \n\nThe following picture shows one possible sequence of operations:\n\n![](https://img.atcoder.jp/wtf19/cff6dc4d81e995e9300ccbaca5bf85de.png)"}]