input: 
  def num_common_substrings(s1: str, s2: str) -> int:
      """
      Given two strings, returns an integer indicating the number of common substrings between them.
      >>> num_common_substrings("abcd", "bae")
      2
      """
      count = 0
      for i in range(len(s1)):
          for j in range(len(s2)):
              if s1[i] == s2[j]:
                  count += 1
      return count
  
output:     count = 0
      for i in range(len(s1)):
          for j in range(len(s2)):
              if s1[i] == s2[j]:
                  count += 1
                  k = 1
                  while i + k < len(s1) and j + k < len(s2) and s1[i + k] == s2[j + k]:
                      count += 1
                      k += 1
      return count