input: Write a Python function to determine the fewest swaps needed to transform one binary string into another.
  assert min_swaps_needed("1101","1110") == 1
  assert min_swaps_needed("111","000") == "Not Possible"
  assert min_swaps_needed("111","110") == "Not Possible"
output: def min_swaps_needed(s1, s2):
      if len(s1) != len(s2):
          return "Not Possible"
      
      count_1_s1 = s1.count('1')
      count_1_s2 = s2.count('1')
  
      if count_1_s1 != count_1_s2:
          return "Not Possible"
  
      swaps = 0
      for i in range(len(s1)):
          if s1[i] != s2[i]:
              swaps += 1
      
      return swaps // 2