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