N = 4 

dp = [0] * (N + 1)

if N >= 1: dp[1] = 1
if N >= 2: dp[2] = 2
if N >= 3: dp[3] = 4

for i in range(4, N + 1):
    dp[i] = dp[i - 1] + dp[i - 2] + dp[i - 3]

print(dp[N])