1
n, m = map(int, input().split())
2
a = sorted([int(x) for x in input().split()])
3
b = sorted([int(x) for x in input().split()])
4
 
5
for v in range(a[-1], b[0]):
6
    if a[0] * 2 <= v:
7
        print(v)
8
        exit(0)
9
 
10
print(-1)