input: from typing import Set
  
  def longest_subsequence(s1: str, s2: str) -> int:
      """Find the length of the longest common subsequence between two strings.
      
      >>> longest_subsequence("ABACEDF", "ABDCF")
      4
      >>> longest_subsequence("ABC", "DEF")
      0
      """
      pass
output:     dp = [[0] * (len(s2) + 1) for _ in range(len(s1) + 1)]
      
      for i in range(1, len(s1) + 1):
          for j in range(1, len(s2) + 1):
              if 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])
      
      return dp[-1][-1]