import std.conv, std.functional, std.range, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.bitmanip, std.complex, std.container, std.math, std.mathspecial, std.numeric, std.regex, std.typecons;
import core.bitop;

class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { if (stdin.eof) { throw new EOFException; } tokens = readln.split; } auto token = tokens.front; tokens.popFront; return token; }
int readInt() { return readToken.to!int; }
long readLong() { return readToken.to!long; }
real readReal() { return readToken.to!real; }

bool chmin(T)(ref T t, in T f) { if (t > f) { t = f; return true; } else { return false; } }
bool chmax(T)(ref T t, in T f) { if (t < f) { t = f; return true; } else { return false; } }

int binarySearch(alias pred, T)(in T[] as) { int lo = -1, hi = cast(int)(as.length); for (; lo + 1 < hi; ) { const mid = (lo + hi) >> 1; (unaryFun!pred(as[mid]) ? hi : lo) = mid; } return hi; }
int lowerBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a >= val)); }
int upperBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a > val)); }


enum E = 61;

void update(ref long t, long f) {
  if (!~t || t > f) {
    t = f;
  }
}

void main() {
  try {
    for (; ; ) {
      const numCases = readInt;
      foreach (caseId; 0 .. numCases) {
        const N = readInt;
        const Y = readLong;
        auto A = new long[N];
        foreach (i; 0 .. N) {
          A[i] = readLong;
        }
        A.sort;
        
        // sort mod 2^e
        auto bss = new long[][](E + 1, N);
        bss[E][] = A[];
        auto mids = new int[E];
        foreach_reverse (e; 0 .. E) {
          const mask = (1L << e) - 1;
          int mid;
          for (; mid < N && !(bss[e + 1][mid] >> e & 1); ++mid) {}
          mids[e] = mid;
          int i = 0, j = mid, k = 0;
          for (; i < mid && j < N; ) {
            bss[e][k++] = ((bss[e + 1][i] & mask) <= (bss[e + 1][j] & mask))
                ? bss[e + 1][i++] : bss[e + 1][j++];
          }
          for (; i < mid; ) bss[e][k++] = bss[e + 1][i++];
          for (; j < N; ) bss[e][k++] = bss[e + 1][j++];
          assert(k == N);
          debug {
            writeln(e, ": ", bss[e]);
          }
        }
        debug {
          writeln("mids = ", mids);
        }
        
        // dp[e][k]: Determined mod 2^e, c carries
        auto dp = new long[][](E + 1, N + 1);
        foreach (e; 0 .. E + 1) {
          dp[e][] = -1;
        }
        dp[0][0] = 0;
        foreach (e; 0 .. E) {
          int num0 = mids[e];
          int num1 = N - mids[e];
          int num2 = 0;
          foreach (c; 0 .. N + 1) {
            if (c > 0) {
              if (bss[e][N - c] >> e & 1) {
                --num1;
                ++num2;
              } else {
                --num0;
                ++num1;
              }
            }
            if (~dp[e][c]) {
              debug {
                if (e < 10) {
                  writeln(e, " ", c, ": ", dp[e][c], "; ", num0, " ", num1, " ", num2);
                }
              }
              // 0
              if ((num1 & 1) == (Y >> e & 1)) {
                update(dp[e + 1][num2], dp[e][c]);
              }
              // 1
              if (((num0 + num2) & 1) == (Y >> e & 1)) {
                update(dp[e + 1][num1 + num2], dp[e][c] | 1L << e);
              }
            }
          }
        }
        
        writeln(dp[E][0]);
      }
    }
  } catch (EOFException e) {
  }
}