input: Write a python function to find the least number of swaps needed to transform one binary string into another.
  assert min_exchange("1101", "1110") == 1
  assert min_exchange("111", "000") == "Not Possible"
  assert min_exchange("111", "110") == "Not Possible"
output: def min_exchange(start, target):
      if len(start) != len(target):
          return "Not Possible"
      
      count_0_to_1 = count_1_to_0 = 0
      for s, t in zip(start, target):
          if s == '0' and t == '1':
              count_0_to_1 += 1
          elif s == '1' and t == '0':
              count_1_to_0 += 1
              
      if count_0_to_1 != count_1_to_0:
          return "Not Possible"
      
      return count_0_to_1