# -*- coding:cp936 -*-

def rabbit(n):
    """
    题目:古典问题:有一对兔子,从出生后第3个月起每个月都生一对兔子,
    小兔子长到第三个月后每个月又生一对兔子,假如兔子都不死,问每个月的兔子总数为多少?
    程序分析:兔子的规律为数列1,1,2,3,5,8,13,21....
    """
    sum = 0
    if n < 3:
        sum = 1
    else:
        sum = rabbit(n-1) + rabbit(n-2)
    return sum

if (__name__ == '__main__'):
    month = int(raw_input("输入月份:"))
    print rabbit(month)