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"
	"errors"
	"fmt"
	"io"
	"math"
	"os"
	"sort"
	"strconv"
)

func main() {
	sc := NewScanner(os.Stdin)
	x, _ := sc.NextInt()
	n, _ := sc.NextInt()
	p, _ := sc.NextInts(n)
	ans := solve(x, n, p)
	fmt.Println(ans)
}

func solve(x, n int, p []int) int {
	sort.Slice(p, func(i, j int) bool { return p[i] < p[j] })
	for i := 0; ; i++ {
		if !contains(p, x-i) {
			return x - i
		} else if !contains(p, x+i) {
			return x + i
		}
	}
}

func contains(p []int, v int) bool {
	n := len(p)
	i := sort.Search(n, func(i int) bool { return p[i] >= v })
	return i < n && p[i] == v
}

// Scanner is a wrapper of bufio.Scanner which is customized for competitive programing.
type Scanner struct {
	bufio.Scanner
}

// NewScanner is a constructor for Scanner.
func NewScanner(r io.Reader) *Scanner {
	sc := Scanner{*bufio.NewScanner(r)}
	sc.Buffer([]byte{}, math.MaxInt64)
	sc.Split(bufio.ScanWords)
	return &sc
}

// NextInt reads a integer from io stream.
func (sc *Scanner) NextInt() (int, error) {
	if !sc.Scan() {
		return -1, errors.New("failed to scan")
	}
	return strconv.Atoi(sc.Text())
}

// NextInts reads n integers from io stream.
func (sc *Scanner) NextInts(n int) (arr []int, e error) {
	arr = make([]int, n)
	for i := 0; i < n; i++ {
		v, e := sc.NextInt()
		if e != nil {
			return nil, e
		}
		arr[i] = v
	}
	return arr, nil
}