CF350A TL

Valera wanted to prepare a Codesecrof round. He's already got one problem and he wants to set a time limit (TL) on it. Valera has written $ n $ correct solutions. For each correct solution, he knows its running time (in seconds). Valera has also wrote $ m $ wrong solutions and for each wrong solution he knows its running time (in seconds). Let's suppose that Valera will set $ v $ seconds TL in the problem. Then we can say that a solution passes the system testing if its running time is at most $ v $ seconds. We can also say that a solution passes the system testing with some "extra" time if for its running time, $ a $ seconds, an inequality $ 2a

The first line contains two integers $ n $ , $ m $ ( $ 1

If there is a valid TL value, print it. Otherwise, print -1.