import math
import sys
def S(): return sys.stdin.readline().rstrip()
def I(): return int(sys.stdin.readline().rstrip())
def MI(): return map(int, sys.stdin.readline().rstrip().split())
def LI(): return list(map(int, sys.stdin.readline().rstrip().split()))
def LS(): return list(sys.stdin.readline().rstrip().split())

n = I()
a = [1, 2]
if n == 1:
    print(1)
elif n == 2:
    print(2)
else:
    for i in range(n-2):
        a.append(a[-1] + a[-2])
    print(a[-1])