3643.上楼梯

#include<stdio.h>
int main()
{
    int i,N;
    int way[21];
    scanf("%d",&N);
    way[1]=1;
    way[2]=2;
    way[3]=4;
    for(i=4;i<21;i++){
        way[i]=way[i-1]+way[i-2]+way[i-3];
    }
    printf("%d",way[N]);
    return 0;
 }