input: def longest_common_subsequence(str1, str2):
      """ Determine the longest common subsequence (LCS) between two given strings.
      The LCS is the longest sequence of characters that appear in the same order in both strings.
      For example, for the strings "ABACEDF" and "ABDCF", the LCS would be "ABCF".
      You should implement an algorithm to return the length of the LCS.
      
      Example:
      >>> longest_common_subsequence("ABACEDF", "ABDCF")
      4  # The length of "ABCF"
      """
output:     m, n = len(str1), len(str2)
      dp = [[0] * (n + 1) for _ in range(m + 1)]
      
      for i in range(1, m + 1):
          for j in range(1, n + 1):
              if str1[i - 1] == str2[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[m][n]