#include<bits/stdc++.h>
#define int long long
using namespace std;
const int N=1000005;
int t,n,a[N];
int h[65][2],sum[65],as,k;
signed main(){
    //  freopen("1.in","r",stdin);
    cin.tie(0),cout.tie(0),ios::sync_with_stdio(0);
    cin>>t;
    while(t--){
    cin>>n>>k;as=0;memset(h,0,sizeof(h));++k;
    memset(sum,0,sizeof(sum));
    for(int i=1;i<=n;++i)cin>>a[i];
    for(int o=1;o<n;++o){
        int x=a[o],y=a[o+1],p,q;
        for(int i=62;i>=0;--i){
            if((1ll<<i)&x)p=1;else p=0;
            if((1ll<<i)&y)q=1;else q=0;
            if(p==q)continue;
            if(p<q)h[i][0]=1;
            else h[i][1]=1;
            break;
        }
    }
    for(int i=62;i>=0;--i)if(h[i][0]&&h[i][1])as=-1;
    if(as==-1){puts("0");continue;}
    sum[0]=(h[0][0]==0&&h[0][1]==0);
    for(int i=1;i<=62;++i)sum[i]=sum[i-1]+(h[i][0]==0&&h[i][1]==0);
    for(int i=62;i>=0;--i)if((1ll<<i)&k){
        if(!h[i][0]&&!h[i][1])as+=((i>=1)?(1ll<<sum[i-1]):1);
        else if(h[i][0]){as+=((i>=1)?(1ll<<sum[i-1]):1);break;}
    }else{
        if(h[i][1])break;
    }
    cout<<as<<"\n";
    }
    return 0;
}