#include <bits/stdc++.h>

using i64 = long long;

constexpr int N = 1E7;
constexpr int inf = 1E9;
int tot;
int trie[N][2];
int f[N];

int newNode() {
    int x = ++tot;
    trie[x][0] = trie[x][1] = 0;
    f[x] = inf;
    return x;
}
void add(int x, int i) {
    int p = 1;
    for (int j = 29; j >= 0; j--) {
        int &q = trie[p][x >> j & 1];
        if (q == 0) {
            q = newNode();
        }
        p = q;
        f[p] = std::min(f[p], i);
    }
}

int query(int a, int b) {
    int ans1 = inf, ans2 = inf;
    int p = 1;
    for (int i = 29; i >= 0; i--) {
        int d = a >> i & 1;
        int e = b >> i & 1;
        if (e) {
            ans1 = std::min(ans1, f[trie[p][d]]);
        } else {
            ans2 = std::min(ans2, f[trie[p][d ^ 1]]);
        }
        p = trie[p][e ^ d];
    }
    ans1 = std::min(ans1, f[p]);
    ans2 = std::min(ans2, f[p]);
    if (ans1 == inf || ans2 == inf) {
        return -1;
    }
    return std::max({1, ans1, ans2});
}

int main() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    
    f[0] = inf;
    
    int n, q;
    std::cin >> n >> q;
    
    tot = 0;
    newNode();
    
    std::vector<int> x(n);
    for (int i = 0; i < n; i++) {
        std::cin >> x[i];
        add(x[i], i);
    }
    
    while (q--) {
        int a, b;
        std::cin >> a >> b;
        std::cout << query(a, b) << "\n";
    }
    
    return 0;
}