简单的动态规划，需要单独判断1,2的情况就行

n = int(input())

a = 2
b = 3
c = 0

if n == 1:
print(2)
elif n == 2:
print(3)
else:
for i in range(n-2):
c = a+b
a = b
b = c
print(c)