input: Write a python function to find the first missing positive number.
  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: def first_Missing_Positive(nums, n):
      for i in range(n):
          while 1 <= nums[i] <= n and nums[nums[i] - 1] != nums[i]:
              nums[nums[i] - 1], nums[i] = nums[i], nums[nums[i] - 1]
      for i in range(n):
          if nums[i] != i + 1:
              return i + 1
      return n + 1