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 Rust (1.42.0) (judge verdict: Accepted)

use std::collections::HashSet;

fn main() {
    let xn = read_vec::<i32>();
    let x = xn[0];
    let n = xn[1];
    if n == 0 {
        println!("{}", x);
        return;
    }

    let v = read_vec::<i32>();
    let mut set: HashSet<i32> = HashSet::new();
    for p in v.into_iter() {
        set.insert(p);
    }

    if !set.contains(&x) {
        println!("{}", x);
        return;
    }

    let mut d = 1;
    let mut ret = 0;
    loop {
        let a = x - d;
        if !set.contains(&a) {
            ret = a;
            break
        }
        let b = x + d;
        if !set.contains(&b) {
            ret = b;
            break;
        }
        d += 1;
    }
    println!("{}", ret);
}

fn read_vec<T: std::str::FromStr>() -> Vec<T> {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim().split_whitespace().map(|e| e.parse().ok().unwrap()).collect()
}