input: Develop a Python function to identify the initial missing positive integer.
  assert first_Missing_Positive([1,2,3,-1,5],5) == 4
  assert first_Missing_Positive([0,-1,-2,1,5,8],6) == 2
  assert first_Missing_Positive([0,1,2,5,-8],5) == 3
output:     
      nums = set(numbers)
      i = 1
      while i in nums:
          i += 1
      return i
  