input: Write a python function to find the shortest substring that can be created by interleaving characters from two given strings.
  assert shortest_substring("abbcd", "accde") == "ac"
  assert shortest_substring("abcdef", "fbdamn") == "bd"
  assert shortest_substring("geeks", "eke") == "ek"
output: def shortest_substring(s1, s2):
      m, n = len(s1), len(s2)
      dp = [[0] * (n + 1) for _ in range(m + 1)]
      
      for i in range(m + 1):
          for j in range(n + 1):
              if i == 0 or j == 0:
                  dp[i][j] = 0
              elif s1[i - 1] == s2[j - 1]:
                  dp[i][j] = dp[i - 1][j - 1] + 1
              else:
                  dp[i][j] = max(dp[i - 1][j], dp[i][j - 1])
      
      lcs_length = dp[m][n]
      if lcs_length == 0:
          return ""
      
      i, j = m, n
      lcs = []
      
      while i > 0 and j > 0:
          if s1[i - 1] == s2[j - 1]:
              lcs.append(s1[i - 1])
              i -= 1
              j -= 1
          elif dp[i - 1][j] > dp[i][j - 1]:
              i -= 1
          else:
              j -= 1
              
      return ''.join(reversed(lcs))