class Solution:
    Memo = {}

    def fib(self, n: int):
        if n in self.Memo:
            return self.Memo[n]
        if n == 0:
            return 0
        if n == 1:
            return 1
        self.Memo[n] = self.fib(n - 1) + self.fib(n - 2)
        return self.Memo[n]