input: Write a python function to find the length of the longest common subsequence of two strings.
  assert compare_strings("AGGTAB", "GXTXAYB") == 4
  assert compare_strings("ABC", "AC") == 2
  assert compare_strings("12345", "54321") == 1
output: def compare_strings(s1, s2):
      m = len(s1)
      b = len(s2)
      dp = [[0] * (b + 1) for _ in range(m + 1)]
  
      for i in range(1, m + 1):
          for j in range(1, b + 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[m][b]