Problem:
Score : 200 points

Problem Statement

There are four towns, numbered 1,2,3 and 4.
Also, there are three roads. The i-th road connects different towns a_i and b_i bidirectionally.
No two roads connect the same pair of towns. Other than these roads, there is no way to travel between these towns, but any town can be reached from any other town using these roads.

Determine if we can visit all the towns by traversing each of the roads exactly once.

Constraints

1 \leq a_i,b_i \leq 4(1\leq i\leq 3)

a_i and b_i are different. (1\leq i\leq 3)

No two roads connect the same pair of towns.

Any town can be reached from any other town using the roads.

Input

Input is given from Standard Input in the following format:

a_1 b_1
a_2 b_2
a_3 b_3

Output

If we can visit all the towns by traversing each of the roads exactly once, print YES; otherwise, print NO.

Sample Input 1

4 2
1 3
2 3

Sample Output 1

YES

We can visit all the towns in the order 1,3,2,4.

Sample Input 2

3 2
2 4
1 2

Sample Output 2

NO

Sample Input 3

2 1
3 2
4 3

Sample Output 3

YES

Solution in Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.List;
import java.util.Arrays;
import java.util.Scanner;
import java.util.ArrayList;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        TaskB solver = new TaskB();
        solver.solve(1, in, out);
        out.close();
    }

    static class TaskB {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            boolean[][] map = new boolean[4][4];

            List<List<Integer>> list = getPermutations(3);
            for (int i = 0; i < 3; i++) {
                int a = in.nextInt();
                int b = in.nextInt();

                map[a - 1][b - 1] = true;
                map[b - 1][a - 1] = true;
            }

            boolean ans = false;

            for (List<Integer> element : list) {
                if (map[element.get(0)][element.get(1)] &&
                        map[element.get(1)][element.get(2)] &&
                        map[element.get(2)][element.get(3)]) {
                    ans = true;
                }
            }

            if (ans) {
                out.print("YES");
            } else {
                out.print("NO");
            }
        }

        private static List<List<Integer>> getPermutations(Integer n) {

            List<List<Integer>> results = new ArrayList<List<Integer>>();

            if (n == 0) {
                results.add(new ArrayList<Integer>(Arrays.asList(0)));
                return results;
            }

            List<List<Integer>> prevResults = getPermutations(n - 1);

            for (List<Integer> permutation : prevResults) {
                for (int i = 0; i <= n; ++i) {
                    permutation.add(i, n);
                    results.add(new ArrayList<Integer>(permutation));
                    permutation.remove(n);
                }
                permutation.clear();
            }
            prevResults.clear();

            return results;
        }

    }
}