class Solution(object):

    def climbStairs(self, n):

        """

        :type n: int

        :rtype: int

        """

        if n <= 2:

            return n

        last =1

        nexttolast = 1

        sums = 0

        for i in range(2,n+1):

            sums = last+nexttolast

            nexttolast = last

            last = sums

        return sums