input: Write a python function to find the number of ways to climb a staircase with n steps where you can take either 1 or 2 steps at a time.
  assert stairs(6) == 13
  assert stairs(4) == 5
  assert stairs(10) == 89
output: def stairs(n):
      if n == 0 or n == 1:
          return 1
      dp = [0] * (n + 1)
      dp[0] = dp[1] = 1
      for i in range(2, n + 1):
          dp[i] = dp[i - 1] + dp[i - 2]
      return dp[n]