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: Accepted)

using System;
using System.Linq;

namespace ABC170
{
    class C_ForbiddenList
    {
        static void Main(string[] args)
        {
            // 1行目の入力
            string[] input1 = Console.ReadLine().Split(' ');
            int x = int.Parse(input1[0]);
            int n = int.Parse(input1[1]);

            if (n == 0)
            {
                Console.WriteLine(x);
                return;
            }

            // 2行目の入力
            string[] input2 = Console.ReadLine().Split(' ');
            int[] p = new int[n];
            for(int i = 0; i < n; i++)
            {
                p[i] = int.Parse(input2[i]);
            }

            // 禁止リストを昇順にソート
            p = p.OrderBy(asc => asc).ToArray();
            // 禁止リストのインデックス
            int index = 0;

            // 答えになる値
            int ans = 0;
            for(int i = 1; i <= 101; i++)
            {
                // 答えになる値と基準値の差の絶対値
                int ansAbs = Math.Abs(x - ans);
                // 現在の値と基準値の差の絶対値
                int currentAbs = Math.Abs(x - i);
                // 現在の値のほうが基準値に近い場合
                if (currentAbs < ansAbs)
                {
                    // 現在の値と禁止リストの値が一致すれば
                    if (index < n && p[index] == i)
                    {
                        // インデックスを加算し、次の値の判定
                        index++;
                        continue;
                    }
                    ans = i;
                }
            }
            Console.WriteLine(ans);
        }
    }
}