input: Write a python function to calculate the Levenshtein distance between two strings.
  assert levenshtein_distance('hello', 'hi') == 4
  assert levenshtein_distance('kitten', 'sitting') == 3
  assert levenshtein_distance('flaw', 'lawn') == 2
output: def levenshtein_distance(s1, s2):
      m, n = len(s1), len(s2)
      dp = [[0] * (n + 1) for _ in range(m + 1)]
      
      for i in range(m + 1):
          dp[i][0] = i
      for j in range(n + 1):
          dp[0][j] = j
      
      for i in range(1, m + 1):
          for j in range(1, n + 1):
              if s1[i - 1] == s2[j - 1]:
                  dp[i][j] = dp[i - 1][j - 1]
              else:
                  dp[i][j] = 1 + min(dp[i - 1][j], dp[i][j - 1], dp[i - 1][j - 1])
      
      return dp[m][n]