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 Go (1.14.1) (judge verdict: Accepted)

package main

import (
	"bufio"
	"fmt"
	"io"
	"os"
)

func main() {
	sc := NewScanner(os.Stdin)
	x, n := sc.Int(), sc.Int()
	p := make(map[int]interface{})

	for i := 0; i < n; i++ {
		p[sc.Int()] = struct{}{}
	}

	for i := 0; ; i++ {
		if _, ok := p[x-i]; !ok {
			fmt.Println(x - i)
			return
		}

		if _, ok := p[x+i]; !ok {
			fmt.Println(x + i)
			return
		}
	}
}

type Scanner struct{ *bufio.Scanner }

func NewScanner(r io.Reader) *Scanner {
	s := bufio.NewScanner(r)
	s.Split(bufio.ScanWords)
	return &Scanner{Scanner: s}
}

func (s *Scanner) next() []byte { s.Scan(); return s.Scanner.Bytes() }

func (s *Scanner) Int() int { return ParseInt(s.next()) }

func ParseInt(b []byte) (n int) {
	var i int
	if b[0] == '-' {
		i++
	}
	for ; i < len(b); i++ {
		n *= 10
		n += int(b[i] - '0')
	}
	if b[0] == '-' {
		return -n
	}
	return
}