input: Write a function to calculate the number of common substrings between two given strings. If there are no common substrings, return 0.
  def num_common_substrings(s1, s2):
      count = 0
      for i in range(len(s1)):
          for j in range(len(s2)):
              if s1[i] == s2[j]:
                  count += 1
      return count
  assert num_common_substrings("abcd", "bae") == 2
  assert num_common_substrings("hello", "world") == 1
  assert num_common_substrings("python", "java") == 2
  assert num_common_substrings("example", "sample") == 1
  assert num_common_substrings("test", "test") == 4
output: def num_common_substrings(s1, s2):
      count = 0
      for i in range(len(s1)):
          for j in range(len(s2)):
              k = 0
              while i + k < len(s1) and j + k < len(s2) and s1[i + k] == s2[j + k]:
                  k += 1
                  count += 1
      return count