Python :

# coding=utf-8
def count_ways(n):
    if n == 1:
        return 1
    elif n == 2:
        return 2
    elif n == 3:
        return 4
    
    dp = [0] * (n + 1)
    dp[1] = 1
    dp[2] = 2
    dp[3] = 4
    
    for i in range(4, n + 1):
        dp[i] = dp[i - 1] + dp[i - 2] + dp[i - 3]
    
    return dp[n]

# 读取输入并处理
while True:
    try:
        n = int(input().strip())
        if n == 0:
            break
        print(count_ways(n))
    except EOFError:
        break