input: from typing import List
  
  def linear_eq(a: int, b: int, c: int) -> int:
      """ Calculate the objective function of a linear equation ax + by = c.
      >>> linear_eq(3, 5, 7)
      0
      """
      return a*x + b*y - c
output:     for x in range(-1000, 1001):
          for y in range(-1000, 1001):
              if a*x + b*y == c:
                  return 0
      return -1