/* package whatever; // don't place package name! */
 
import java.util.*;
import java.lang.*;
import java.io.*;
 
/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
	public static void main (String[] args) throws java.lang.Exception
	{
		// your code goes here
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		System.out.print(climbstairs(N));
	}
	public static int climbstairs(int N){
		int[] arr = new int[N+1];
		arr[1] = 1;
		arr[2] = 2;
		arr[3] = 4;
		for(int i=4;i<N+1;i++){
			arr[N] = arr[N-1]+arr[N-2]+arr[N-3];
		}
		return arr[arr.length-1];
	}
}

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NA==

4