from sys import stdin, stdout

def upStairs(n):
    dp = [0]*(n+1)
    dp[1], dp[2] = 1, 2
    for i in range(3, n+1): dp[i] = dp[i-1]+dp[i-2]
    return dp[n]

n = int(stdin.readline().replace("\n",""))
stdout.writelines("%d\n" % upStairs(n))