题解 | #兔子的数量#

x1 = 2
x2 = 3
y = int(input())

if(y == 1):
    print(x1)
elif(y == 2):
    print(x2)
else:
    for i in range(y - 2):
        a = x1 + x2
        x1 = x2
        x2 = a
    print(a)