def solve_mini(N,A,Y,M=60):
    dp = [[1<<(M+1) for j in range(N+1)] for i in range(M+1)]
    for j in range(Y>>M,N+1,2):
        dp[M][j] = 0
    
    low_sort = [[i for i in range(N)] for _ in range(M)]
    
    """
    for t in range(M):
        low_sort[t].sort(key=lambda i:A[i]%(1<<(t+1)),reverse=True)
    """
    
    pre = [i for i in range(N)]
    for t in range(M):
        zero,one = [],[]
        for i in pre:
            if A[i]>>t & 1:
                one.append(i)
            else:
                zero.append(i)
        pre = one + zero
        low_sort[t] = [i for i in pre]
    
    for i in range(M)[::-1]:
        zero,one,two = 0,0,0
        for j in range(N):
            if A[j]>>i & 1:
                one += 1
            else:
                zero += 1
        
        if one&1==(Y>>i & 1):
            dp[i][0] = min(dp[i][0], 2*dp[i+1][two])
        if (zero+two)&1==(Y>>i & 1):
            dp[i][0] = min(dp[i][0], 2*dp[i+1][one+two]+1)
        
        if i==0:
            break

        for j in range(1,N+1):
            next_up = low_sort[i-1][j-1]
            if A[next_up]>>i & 1:
                one -= 1
                two += 1
            else:
                zero -= 1
                one += 1

            if j!=N:
                next_next_up = low_sort[i-1][j]
                if A[next_up]%(1<<(t+1)) == A[next_next_up]%(1<<(t+1)):
                    continue
            
            if one&1==(Y>>i & 1):
                dp[i][j] = min(dp[i][j], 2*dp[i+1][two])
            if (zero+two)&1==(Y>>i & 1):
                dp[i][j] = min(dp[i][j], 2*dp[i+1][one+two]+1)
            
    
    if dp[0][0]==1<<(M+1):
        return -1
    return dp[0][0]

def brute(N,A,Y):
    ans = 2**61
    def dfs(d,tmp,up):
        nonlocal ans
        if d==61:
            if len(up)&1==0:
                return tmp
            else:
                return 1<<61
        res = ans
        if tmp > res:
            return ans

        zero,one,two = 0,set(),set()
        for i in range(N):
            if A[i]>>d & 1:
                if i in up:
                    two.add(i)
                else:
                    one.add(i)
            else:
                if i in up:
                    one.add(i)
                else:
                    zero += 1
        
        if len(one)&1==(Y>>d)&1:
            res = min(res,dfs(d+1,tmp,two))
            if res != 2**61:
                return res
        if (zero+len(two))&1==(Y>>d)&1 and res > tmp + (1<<d):
            n_up = set([i for i in one]+[j for j in two])
            res = min(res,dfs(d+1,tmp+(1<<d),n_up))
        
        ans = min(ans,res)
        
        return res

    res = dfs(0,0,set())
    if res < 2**61:
        return res
    return -1

for _ in range(int(input())):
    N,Y = map(int,input().split())
    A = list(map(int,input().split()))
    print(brute(N,A,Y))