Problem:
Score : 100 points

Problem Statement

You are given four digits N_1, N_2, N_3 and N_4. Determine if these can be arranged into the sequence of digits "1974".

Constraints

0 \leq N_1, N_2, N_3, N_4 \leq 9

N_1, N_2, N_3 and N_4 are integers.

Input

Input is given from Standard Input in the following format:

N_1 N_2 N_3 N_4

Output

If N_1, N_2, N_3 and N_4 can be arranged into the sequence of digits "1974", print YES; if they cannot, print NO.

Sample Input 1

1 7 9 4

Sample Output 1

YES

We can get 1974 by swapping N_2 and N_3.

Sample Input 2

1 9 7 4

Sample Output 2

YES

We already have 1974 before doing anything.

Sample Input 3

1 2 9 1

Sample Output 3

NO

Sample Input 4

4 9 0 8

Sample Output 4

NO

Solution in C# (Mono 4.6.2.0) (judge verdict: Wrong Answer)

using System;

namespace AtCoder {
    class Program {
        static void Main(string[] args) {
            var inputs = Console.ReadLine().Split(' ');
            bool[] flags = new bool[4] { false, false, false,false};

            foreach(var s in inputs)
            {
                switch (s)
                {
                    case "1":
                        if (!flags[0])
                        {
                            flags[0] = true;
                        } else
                        {
                            Console.WriteLine(No);
                            return;
                        }
                        break;
                    case "9":
                        if (!flags[1])
                        {
                            flags[1] = true;
                        } else
                        {
                            Console.WriteLine(No);
                            return;
                        }
                        break;
                    case "7":
                        if (!flags[2])
                        {
                            flags[2] = true;
                        } else
                        {
                            Console.WriteLine(No);
                            return;
                        }
                        break;
                    case "4":
                        if (!flags[3])
                        {
                            flags[3] = true;
                        } else
                        {
                            Console.WriteLine(No);
                            return;
                        }
                        break;
                    default:
                        Console.WriteLine(No);
                        return;
                        
                }

            }
            Console.WriteLine(Yes);
            return;
        }

        static string No = "NO";
        static string Yes = "Yes";

    }
}