using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine((Isshuffle("abc", "def", "dabecf")));

        Console.WriteLine((Isshuffle("bac", "def", "dabecf")));

        Console.WriteLine((Isshuffle("otto", "anna", "")));

        Console.WriteLine((Isshuffle("otto", "anna", "oatntnoa")));

        //Console.WriteLine("Hello World");

    }

    public static bool Isshuffle(string s1, string s2, string res){

        char[] a1 = s1.ToCharArray();

        char[] a2 = s2.ToCharArray();

        int p1 = 0;

        int p2 = 0; 

        if(res != ""){

        foreach(var a in res){

            if(p1<a1.Length && a == a1[p1]){

                p1++;

            }else if(p2<a2.Length && a == a2[p2]){

                p2++;

            }else{

            return false;

            }

        }

​

            return true;

        }else{

            return false;

        }

    }

}