// ---------- begin chmin, chmax ----------
pub trait ChangeMinMax {
    fn chmin(&mut self, x: Self) -> bool;
    fn chmax(&mut self, x: Self) -> bool;
}

impl<T: PartialOrd> ChangeMinMax for T {
    fn chmin(&mut self, x: Self) -> bool {
        *self > x && {
            *self = x;
            true
        }
    }
    fn chmax(&mut self, x: Self) -> bool {
        *self < x && {
            *self = x;
            true
        }
    }
}
// ---------- end chmin, chmax ----------
// ---------- begin scannner ----------
#[allow(dead_code)]
mod scanner {
    use std::str::FromStr;
    pub struct Scanner<'a> {
        it: std::str::SplitWhitespace<'a>,
    }
    impl<'a> Scanner<'a> {
        pub fn new(s: &'a String) -> Scanner<'a> {
            Scanner {
                it: s.split_whitespace(),
            }
        }
        pub fn next<T: FromStr>(&mut self) -> T {
            self.it.next().unwrap().parse::<T>().ok().unwrap()
        }
        pub fn next_bytes(&mut self) -> Vec<u8> {
            self.it.next().unwrap().bytes().collect()
        }
        pub fn next_chars(&mut self) -> Vec<char> {
            self.it.next().unwrap().chars().collect()
        }
        pub fn next_vec<T: FromStr>(&mut self, len: usize) -> Vec<T> {
            (0..len).map(|_| self.next()).collect()
        }
    }
}
// ---------- end scannner ----------

use std::io::Write;
use std::collections::*;

type Map<K, V> = BTreeMap<K, V>;

fn main() {
    use std::io::Read;
    let mut s = String::new();
    std::io::stdin().read_to_string(&mut s).unwrap();
    let mut sc = scanner::Scanner::new(&s);
    let out = std::io::stdout();
    let mut out = std::io::BufWriter::new(out.lock());
    run(&mut sc, &mut out);
}

fn run<W: Write>(sc: &mut scanner::Scanner, out: &mut std::io::BufWriter<W>) {
    let t: u32 = sc.next();
    for _ in 0..t {
        let n: usize = sc.next();
        let y: u64 = sc.next();
        let mut a = sc.next_vec::<u64>(n);
        a.sort();
        let mut dp = Map::new();
        dp.insert(a, 0);
        for i in 0..61 {
            let y = y >> i & 1;
            let mut next = Map::new();
            for (a, v) in dp {
                for add in 0..2 {
                    let v = v + (add << i);
                    let mut a = a.iter().map(|a| *a + add).collect::<Vec<_>>();
                    if a.iter().fold(0, |s, a| s ^ *a) & 1 == y {
                        a.iter_mut().for_each(|a| *a /= 2);
                        next.entry(a).or_insert(std::u64::MAX).chmin(v);
                    }
                }
            }
            dp = next;
        }
        if let Some(v) = dp.iter().filter(|p| p.0.iter().all(|p| *p == 0)).map(|p| *p.1).min() {
            writeln!(out, "{}", v).ok();
        } else {
            writeln!(out, "-1").ok();
        }
    }
}