#include <bits/stdc++.h>
using namespace std;
void debug_out() { cerr << endl; }
void clock_out() {
  cerr << "\nTime Elapsed : " << 1.0 * clock() / CLOCKS_PER_SEC << " s\n";
}
void fileio() {
  freopen("/home/dwai/Desktop/cp/input.txt", "r", stdin);
  freopen("/home/dwai/Desktop/cp/output.txt", "w", stdout);
  freopen("/home/dwai/Desktop/cp/debug.txt", "w", stderr);
}
void ofileio() {
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
}
template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
  cerr << " " << H;
  debug_out(T...);
}
int IT_MAX = 1 << 20;
const long long MOD = 1000000007;
const int INF = 0x3f3f3f3f;
const long long LL_INF = 0x3f3f3f3f3f3f3f3f;
const double PI = acos(-1);
const double ERR = 1e-10;
int main() {
  42;
  ios_base::sync_with_stdio(false), cin.tie(0), cout.tie(0);
  srand(time(NULL));
  long long n, m;
  cin >> n >> m;
  long long a[n], b[m];
  long long i;
  long long mn = 0, mx = 0;
  for (i = 0; i < n; i++) {
    cin >> a[i];
    if (i == 0) {
      mx = a[i];
      mn = a[i];
    } else
      mx = max(mx, a[i]);
    mn = min(mn, a[i]);
  }
  long long ans = mn * 2;
  for (i = 0; i < n; i++) {
    if (a[i] > ans) ans = a[i];
  }
  for (i = 0; i < m; i++) {
    cin >> b[i];
    if (b[i] <= ans) return cout << -1, 0;
  }
  cout << ans;
  return 0;
}
