# Greatest Common Divisor
# C4B:    |[0, x]| + |[0, y]|
# ecoimp: [x | x ≥ 0] + [y | y ≥ 0]

def gcd():
  var x, y

  while x > 0 and y > 0:
    tick 1
    if x > y:
      x = x - y
    else:
      y = y - x