#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<ll,ll> P;
#define fi first
#define se second
#define mkp make_pair
#define pb emplace_back
#define popcnt __builtin_popcountll
const ll mod = 998244353;
inline ll read(){
	ll x=0, f=1; char ch=getchar();
	while(ch<'0' || ch>'9') { if(ch=='-') f=-1; ch=getchar(); }
	while(ch>='0' && ch<='9') x=x*10+ch-'0', ch=getchar();
	return x*f;
}
inline int lg2(int x){ return 31^__builtin_clz(x); }
inline ll lg2(ll x){ return 63^__builtin_clzll(x); }
inline ll qpow(ll a,ll b){
	ll ans=1, base=a;
	while(b){
		if(b&1) ans=ans*base%mod;
		base=base*base%mod; b>>=1;
	}
	return ans;
}
void init(){ }
const int M = 30;

int n,q,x[300005],tr[9000005],cnt=1;
int son[9000005][2];

void add(int pos,int id){ tr[pos]=min(tr[pos],id); }

void input(int x,int id){
	int rt=1; add(rt, id);
	for(int i=(M-1);i>=0;i--){
		int w=((x>>i)&1);
		if(!son[rt][w]) son[rt][w]=(++cnt);
		// cerr<<"rt = "<<rt<<endl;
		rt=son[rt][w];
		add(rt, id);
	}
}

void procedure(){
	memset(tr, 0x3f, sizeof(tr));
	n=read(),q=read();
	for(int i=1;i<=n;i++) x[i]=read(), input(x[i], i);

	// cerr<<"zhenghanyun2"<<endl;
	while(q--){
		int a=read(), b=read();
		if((a^x[1])==b) {puts("1"); continue;}
		else if((a^x[1])<b){
			a^=((1<<M)-1),b^=((1<<M)-1);
			// find (a^x[1])>=b
			int ans=0x3f3f3f3f, rt=1;
			for(int i=(M-1);i>=0;i--){
				int w=(b>>i)&1, q=((a>>i)&1);
				if(w){
					ans=min(ans, tr[son[rt][q]]);
					// cout<<"get "<<tr[son[rt][q]]<<endl;
				}
				rt=son[rt][w^q];
				if(!rt) break;
			}
			ans=min(ans, tr[rt]);
			if(ans>n){ puts("-1"); continue; }
			else printf("%d\n", ans-1);
		}else{
			// find (a^x[1])>=b
			int ans=0x3f3f3f3f, rt=1;
			for(int i=(M-1);i>=0;i--){
				int w=(b>>i)&1, q=w^((a>>i)&1);
				if(w) ans=min(ans, tr[son[rt][q^1]]);
				rt=son[rt][q];
				if(!rt) break;
			}
			ans=min(ans, tr[rt]);
			if(ans>n){ puts("-1"); continue; }
			else printf("%d\n", ans-1);
		}
	}
}

int main(){
	#ifdef LOCAL
		assert(freopen("input.txt","r",stdin));
		assert(freopen("output.txt","w",stdout));
	#endif
	ll T=1;
	init();
	while(T--) procedure();
	return 0;
}