# -*- coding: utf-8 -*-

N = int(input())

dp = [-1]*(N+1)
def goal(i,j):
    if dp[i] >= 0:
        return dp[i]

    pattern_sum = -1
    if i == j:
        pattern_sum = 0
    elif i == (j-1):
        pattern_sum = 1
    elif i == (j-2):
        pattern_sum = 2
    else:
        pattern_sum = goal(i+1,j) + goal(i+2,j)
    
    dp[i] = pattern_sum
    return dp[i]

print(goal(0,N))