题解 | #兔子的数量#

#解法一
num = int(input())
i = 1
a = [1,2]
while True:
    if(i >= num):
        print(a[i])
        break
    else:
        i = i+1
        b = a[i-1]+a[i-2]
        a.append(b)
		
		
###解法二
def fun(n):
    a=0
    if(n == 1):
        a = 2
    elif(n == 2):
        a = 3
    else:
        a = fun(n-1)+fun(n-2)
    return a

num = int(input())
if(num>=10):
    print('Error')
else:
    print(fun(num))