cf 350A

#include<bits\stdc++.h>

using namespace std;

int main()
{
    int n, m;
    scanf("%d %d", &n, &m);
    int i, a[n], b[m];
    for(i = 0; i < n; i++) scanf("%d", &a[i]);
    for(i = 0; i < m; i++) scanf("%d", &b[i]);

    int x = *min_element(a, a+n);
    int y = *max_element(a, a+n);
    int z = *min_element(b, b+m);
    //int pro_ans = max(2*x, y);

	if(max(2*x, y) < z) printf("%d\n", max(2*x, y));
	else printf("-1\n");
}