import math
import sys
from operator import itemgetter
input = sys.stdin.readline

# a = int(input().rstrip())
# a, b = map(int, input().rstrip().split())
# D = list(map(int, input().rstrip().split()))
# H = [int(input().rstrip()) for _ in range(L)]
# DD = [[0 for j in range(100)] for i in range(200)]

# if __name__ == '__main__':

N = int(input().rstrip())
dp = [0] * (N+1)
dp[1] = 1
dp[2] = 2

for i in range(2, N):
	dp[i+1] = dp[i] + dp[i-1]

print(dp[N])