while True:
    try:
        #获取输入数据
        data1 = int(input())
        #设置初值
        #一个月兔子个数
        N1 = 1
        #两个月兔子个数
        N2 = 0
        #三个月以及三个月以上兔子个数
        N3 = 0
        for n in range(1,data1):
            N3 = N3 + N2
            N2 = N1
            N1 = N3
        print(N1+N2+N3)
       
        
    except:
        break