#include<bits/stdc++.h> #define fi first #define se second #define int long long using namespace std; const int N=30000010; int a[N],b[N]; int trie[N][2]; int num[N],tot; void ins(int x,int id) { int p=0; for(int i=30;i>=0;i--) { bool c=(x>>i)&1; if(!trie[p][c]) trie[p][c]=++tot; p=trie[p][c]; } if(!num[p]) num[p]=id; } pair<int,int> findmin(int x) { int p=0,now=0; for(int i=30;i>=0;i--) { bool c=(x>>i)&1; if(trie[p][c]) p=trie[p][c]; else p=trie[p][1-c],now+=(1<<i); } return make_pair(now,num[p]); } pair<int,int> findmax(int x) { int p=0,now=0; for(int i=30;i>=0;i--) { bool c=(x>>i)&1; if(trie[p][1-c]) p=trie[p][1-c],now+=(1<<i); else p=trie[p][c]; } return make_pair(now,num[p]); } int find(int l,int r,int x,int y) { int ans=0; if(l<r) { while(l<=r) { int mid=(l+r)/2; if((a[mid]^x)>y) ans=mid,r=mid-1; else l=mid+1; } return ans-1; } else { swap(l,r); while(l<=r) { int mid=(l+r)/2; if((a[mid]^x)>y) ans=mid,l=mid+1; else r=mid-1; } return ans; } } signed main() { //freopen("my.in","r",stdin); //freopen("my.out","w",stdout); ios::sync_with_stdio(false); cin.tie(0);cout.tie(0); int n,q; cin>>n>>q; for(int i=1;i<=n;i++) cin>>a[i],ins(a[i],i); while(q--) { int x,y; cin>>x>>y; pair<int,int> l=findmin(x),r=findmax(x); //cout<<l.first<<" "<<r.first<<'\n'; if(r.first<y||l.first>y) cout<<-1<<'\n'; else cout<<find(l.second,r.second,x,y)<<'\n'; //cout<<ans<<'\n'; } return 0; }