#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m;
  cin >> n >> m;
  int cor[n], wro[m];
  for (int i = 0; i < n; i++) cin >> cor[i];
  for (int i = 0; i < m; i++) cin >> wro[i];
  int tl;
  sort(cor, cor + n);
  sort(wro, wro + m);
  int v = max(2 * cor[0], cor[n - 1]);
  if (v < wro[0])
    cout << v;
  else
    cout << "-1";
  return 0;
}
