Problem:
Score : 300 points

Problem Statement

Given are an integer X and an integer sequence of length N: p_1, \ldots, p_N.

Among the integers not contained in the sequence p_1, \ldots, p_N (not necessarily positive), find the integer nearest to X, that is, find the integer whose absolute difference with X is the minimum. If there are multiple such integers, report the smallest such integer.

Constraints

1 \leq X \leq 100

0 \leq N \leq 100

1 \leq p_i \leq 100

p_1, \ldots, p_N are all distinct.

All values in input are integers.

Input

Input is given from Standard Input in the following format:

X N
p_1 ... p_N

Output

Print the answer.

Sample Input 1

6 5
4 7 10 6 5

Sample Output 1

8

Among the integers not contained in the sequence 4, 7, 10, 6, 5, the one nearest to 6 is 8.

Sample Input 2

10 5
4 7 10 6 5

Sample Output 2

9

Among the integers not contained in the sequence 4, 7, 10, 6, 5, the ones nearest to 10 are 9 and 11. We should print the smaller one, 9.

Sample Input 3

100 0

Sample Output 3

100

When N = 0, the second line in the input will be empty. Also, as seen here, X itself can be the answer.

Solution in C# (.NET Core 3.1.201) (judge verdict: Wrong Answer)

using System;
using System.Linq;
using System.Collections.Generic;

namespace Atcoder
{
    class Program
    {
        public static void Main(string[] args)
        {
            int[] input1 = Console.ReadLine().Split(" ").Select(i => int.Parse(i)).ToArray();
            int[] input2 = { 0 };
            if (input1[1] != 0)
            {
                input2 = Console.ReadLine().Split(" ").Select(i => int.Parse(i)).ToArray();
            }
            else
                Console.Write(input1[0]);

            var plusList = new List<long>();
            var minusList = new List<long>();
            foreach(var num in input2) 
            {
                var a = input1[0] - num;
                if (a >= 0)
                    plusList.Add(a);
                else
                {
                    minusList.Add(a);
                }
            };
            plusList.Sort();
            minusList.Sort();
            minusList.Reverse();
            int b = 0;
            foreach(var num in plusList) 
            {
                if (num != b)
                    break;
                b++;
            }
            var ansa = input1[0] - b;
            int c = -1;
            foreach(var num in minusList)
            {
                if (num != c)
                    break;
                c--;
            }
            var ansb = input1[0] - c;

            if (Math.Abs(input1[0] - ansa) == Math.Abs(input1[0] - ansb))
                Console.Write(Math.Min(ansa, ansb));

            if (Math.Abs(input1[0] - ansa) > Math.Abs(input1[0] - ansb))
                Console.Write(ansb);
            else
                Console.Write(ansa);

           
        }
    }
}