input: from typing import List
  
  
  def num_common_substrings(s1: str, s2: str) -> int:
      """ Find the number of common substrings between two given strings.
      >>> num_common_substrings("abcd", "bae")
      3
      """
output:     
      count = 0
      set1 = set()
      set2 = set()
      
      for i in range(len(s1)):
          for j in range(i + 1, len(s1) + 1):
              set1.add(s1[i:j])
      
      for i in range(len(s2)):
          for j in range(i + 1, len(s2) + 1):
              set2.add(s2[i:j])
      
      common_substrings = set1 & set2
      count = len(common_substrings)
      
      return count
  