#include<bits/stdc++.h>
using namespace std;
struct time_helper{
#ifdef LOCAL
clock_t time_last;time_helper(){time_last=clock();}void test(){auto time_now=clock();std::cerr<<"time:"<<1.*(time_now-time_last)/CLOCKS_PER_SEC<<";all_time:"<<1.*time_now/CLOCKS_PER_SEC<<std::endl;time_last=time_now;}~time_helper(){test();}
#else
void test(){}
#endif
}time_helper;
#ifdef LOCAL
#include"dbg.h"
#else
#define dbg(...) (__VA_ARGS__)
#endif
namespace Fread{const int SIZE=1<<16;char buf[SIZE],*S,*T;inline char getchar(){if(S==T){T=(S=buf)+fread(buf,1,SIZE,stdin);if(S==T)return'\n';}return *S++;}}namespace Fwrite{const int SIZE=1<<16;char buf[SIZE],*S=buf,*T=buf+SIZE;inline void flush(){fwrite(buf,1,S-buf,stdout);S=buf;}inline void putchar(char c){*S++=c;if(S==T)flush();}struct NTR{~NTR(){flush();}}ztr;}
#define getchar Fread::getchar
#define putchar Fwrite::putchar
int print_precision=10;bool print_T_endl=1;char print_between=' ';
template<typename T>struct is_char{static constexpr bool value=(std::is_same<T,char>::value||std::is_same<T,signed char>::value||std::is_same<T,unsigned char>::value);};template<typename T>struct is_integral_ex{static constexpr bool value=(std::is_integral<T>::value||std::is_same<T,__int128>::value)&&!is_char<T>::value;};template<typename T>struct is_floating_point_ex{static constexpr bool value=std::is_floating_point<T>::value||std::is_same<T,__float128>::value;};namespace Fastio{struct Reader;struct Writer;template<size_t id>struct read_tuple{template<typename...T>static void read(Reader&stream,std::tuple<T...>&x){read_tuple<id-1>::read(stream,x);stream>>get<id-1>(x);}};template<>struct read_tuple<0>{template<typename...T>static void read([[maybe_unused]]Reader&stream,[[maybe_unused]]std::tuple<T...>&x){}};template<size_t id>struct print_tuple{template<typename...T>static void print(Writer&stream,const std::tuple<T...>&x){print_tuple<id-1>::print(stream,x);putchar(print_between);stream<<get<id-1>(x);}};template<>struct print_tuple<1>{template<typename...T>static void print(Writer&stream,const std::tuple<T...>&x){stream<<get<0>(x);}};template<>struct print_tuple<0>{template<typename...T>static void print([[maybe_unused]]Writer&stream,[[maybe_unused]]const std::tuple<T...>&x){}};
struct Reader{template<typename T>typename std::enable_if_t<std::is_class<T>::value,Reader&>operator>>(T&x){for(auto &y:x)*this>>y;return *this;}template<typename...T>Reader&operator>>(std::tuple<T...>&x){read_tuple<sizeof...(T)>::read(*this,x);return *this;}template<typename T>typename std::enable_if_t<is_integral_ex<T>::value,Reader&>operator>>(T&x){char c=getchar();short f=1;while(c<'0'||c>'9'){if(c=='-')f*=-1;c=getchar();}x=0;while(c>='0'&&c<='9'){x=(x<<1)+(x<<3)+(c^48);c=getchar();}x*=f;return *this;}template<typename T>typename std::enable_if_t<is_floating_point_ex<T>::value,Reader&>operator>>(T&x){char c=getchar();short f=1,s=0;x=0;T t=0;while((c<'0'||c>'9')&&c!='.'){if(c=='-')f*=-1;c=getchar();}while(c>='0'&&c<='9'&&c!='.')x=x*10+(c^48),c=getchar();if(c=='.')c=getchar();else return x*=f,*this;while(c>='0'&&c<='9')t=t*10+(c^48),s++,c=getchar();while(s--)t/=10.0;x=(x+t)*f;return*this;}template<typename T>typename std::enable_if_t<is_char<T>::value,Reader&>operator>>(T&c){c=getchar();while(c=='\n'||c==' '||c=='\r')c=getchar();return *this;}Reader&operator>>(char*str){int len=0;char c=getchar();while(c=='\n'||c==' '||c=='\r')c=getchar();while(c!='\n'&&c!=' '&&c!='\r')str[len++]=c,c=getchar();str[len]='\0';return*this;}template<typename T1,typename T2>Reader&operator>>(std::pair<T1,T2>&x){*this>>x.first>>x.second;return *this;}Reader&operator>>(std::string&str){str.clear();char c=getchar();while(c=='\n'||c==' '||c=='\r')c=getchar();while(c!='\n'&&c!=' '&&c!='\r')str.push_back(c),c=getchar();return*this;}Reader(){}}cin;const char endl='\n';
struct Writer{typedef __int128 mxdouble;template<typename T>typename std::enable_if_t<std::is_class<T>::value,Writer&>operator<<(const T&x){for(auto q:x){*this<<q;if(!is_class<decltype(q)>::value)*this<<print_between;}if(!is_class<typename T::value_type>::value&&print_T_endl)*this<<'\n';return *this;}template<typename...T>Writer&operator<<(const std::tuple<T...>&x){print_tuple<sizeof...(T)>::print(*this,x);if(print_T_endl)*this<<'\n';return *this;}template<typename T>typename std::enable_if_t<is_integral_ex<T>::value,Writer&>operator<<(T x){if(x==0)return putchar('0'),*this;if(x<0)putchar('-'),x=-x;static int sta[45];int top=0;while(x)sta[++top]=x%10,x/=10;while(top)putchar(sta[top]+'0'),--top;return*this;}template<typename T>typename std::enable_if_t<is_floating_point_ex<T>::value,Writer&>operator<<(T x){if(x<0)putchar('-'),x=-x;x+=pow(10,-print_precision)/2;mxdouble _=x;x-=(T)_;static int sta[45];int top=0;while(_)sta[++top]=_%10,_/=10;if(!top)putchar('0');while(top)putchar(sta[top]+'0'),--top;putchar('.');for(int i=0;i<print_precision;i++)x*=10;_=x;while(_)sta[++top]=_%10,_/=10;for(int i=0;i<print_precision-top;i++)putchar('0');while(top)putchar(sta[top]+'0'),--top;return*this;}template<typename T>typename std::enable_if_t<is_char<T>::value,Writer&>operator<<(const T&c){putchar(c);return*this;}Writer&operator<<(char*str){int cur=0;while(str[cur])putchar(str[cur++]);return *this;}Writer&operator<<(const char*str){int cur=0;while(str[cur])putchar(str[cur++]);return*this;}template<typename T1,typename T2>Writer&operator<<(const std::pair<T1,T2>&x){*this<<x.first<<print_between<<x.second;if(print_T_endl)*this<<'\n';return *this;}Writer&operator<<(const std::string&str){int st=0,ed=str.size();while(st<ed)putchar(str[st++]);return*this;}Writer(){}}cout;}
#define cin Fastio::cin
#define cout Fastio::cout
#define endl Fastio::endl

constexpr int maxn=3e5+5;
int n,q,a[maxn];
int ch[maxn*30][2],tot=1;
int id[maxn*30];
int main(){
    cin>>n>>q;
    unordered_map<int,int> mp;
    for(int i=1;i<=n;i++){
        cin>>a[i];
        int p=1;
        for(int j=29;j>=0;j--){
            int&c=ch[p][a[i]>>j&1];
            if(!c) c=++tot;
            p=c;
        }id[p]=i;
        mp[a[i]]=i;
    }
    while(q--){
        int A,B;cin>>A>>B;
        if(mp.count(B^A)){
            int w=mp[B^A];
            cout<<w-(w==n)<<'\n';
            continue;
        }
        auto f=[&](int x){return (a[x]^A)<=B;};
        auto get=[&](int A){
            int p=1;
            for(int i=29;i>=0;i--){
                int x=(A>>i&1);
                p=ch[p][x^1]?:ch[p][x];
            }
            return id[p];
        };
        int l=1,r=n;
        if(f(l)==f(r)) r=get(f(l)?A:~A);
        if(f(l)==f(r)){cout<<"-1\n";continue;}
        while(l+1<r){
            int mid=l+r>>1;
            if(f(mid)==f(l)) l=mid;
            else r=mid;
        }
        cout<<l<<'\n';
    }
}