n=int(input())
a=[0 for i in range(n+1)]
for i in range(1,n+1):
    if(i==1):
        a[1]=1
    elif(i==2):
        a[2]=2
    elif(i==3):
        a[3]=4
    else:
        a[i]=a[i-1]+a[i-2]+a[i-3]
print(a[n])
/**************************************************************
	Problem: 1367
	User: admin
	Language: Python
	Result: Accepted
	Time:214 ms
	Memory:34480 kb
****************************************************************/