Water#609
Recently, Sergey went to the well for water but did not return. He took canisters with him, each of which he filled completely with water. Now Sergey wants to deliver them to his country house. This is where the problem lies. At one time, Sergey can carry no more than canisters, because he has only two hands. Moreover, he can carry no more than liters of water.
Now Sergey stands at the well and wonders what is the minimum number of trips he needs to take all the water home, and whether he can do it at all. Help him solve this problem.
The first line contains two integers and (1 \le n \le 10^5). The second line contains integers — the volumes of the canisters in liters. All input numbers are positive and do not exceed 10^9.
If Sergey cannot take all the water home, print Impossible. Otherwise, print a single number — the minimum required number of trips.
Example 1