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.Collections.Generic;
using System.Linq;
using System.Security;
using System.Text;
using System.Threading.Tasks;

namespace AtCoder
{
    class Program
    {
        static void Main(string[] args)
        {
            //1行目の入力の読み込み
            var line1 = Console.ReadLine();
            var firstLine = line1.Split();

            //答えを格納する変数を宣言
            int answer = 0;

            //1行目の読み込みの一つ目の数をNに代入
            int N = int.Parse(firstLine[0]);

            //処理の回数を入れる変数を初期化
            int count = 1;

            //2行目の入力を格納する配列を定義
            string[] secondLine = new string[int.Parse(firstLine[1])];


            //本題の処理
            if(firstLine[1] == "0") //2行目で渡される数がなければanswerにNを代入
            {
                answer = N;
                var trash = Console.ReadLine();
            }
            else
            {
                //2行目の入力の読み込み
                var line2 = Console.ReadLine();
                secondLine = line2.Split();

                //secondLineを昇順にソート
                Array.Sort(secondLine);

                /*//firstLine[0]がsecondLineになければanswerにfirstLine[0]をいれる
                if (Method(secondLine, firstLine[0]) == -1)
                {
                    answer = N;
                }
                else
                {*/
                //答えが見つかるまで探す
                while (true)
                {
                    /*
                     * Nからcount（初期値は1）を引いた数で探す
                     * countは処理回数ごとに1増え、正負が反転する
                     */
                    if (Method(secondLine, $"{N - count}") == -1)
                    {
                        answer = N - count;
                        break;
                    }
                    else
                    {
                        N -= count;
                        count++;
                        count *= -1;
                        continue;
                    }
                }
                //}

            }
            Console.WriteLine(answer);                                 
        }

        //与えられた配列から、与えられた文字列を探しその配列番号を返すメソッド
        public static int Method(string[] line, string str)
        {
            return Array.IndexOf(line, str);
        }
    }
}