1502  Slalom
Time Limit : 5 Second
Memory Limit : 256 MB
Submission: 13
Solved: 4
 Description
 You are competing in a ski slalom, and you need to select the best skis for the race. The format of the race is that there are N pairs of left and right gates, where each right gate is W metres to the right of its corresponding left gate, and you may neither pass to the left of the left gate nor to the right of the right gate. The i^{th} pair of gates occurs at distance y_{i} down the hill, with the horizontal position of the i^{th} left gate given by x_{i}. Each gate is further down the hill than the previous gate (i.e. y_{i} < y_{i+1} for all i).
You may select from S pairs of skis, where the j^{th} pair has speed s_{j}. Your movement is governed by the following rule: if you select a pair of skis with speed s_{j}, you move with a constant downward velocity of s_{j} metres per second. Additionally, at any time you may move at a horizontal speed of at most v_{h} metres per second.
You may start and finish at any two horizontal positions. Determine which pair of skis will allow you to get through the race course, passing through all the gates, in the shortest amount of time.  Input
 The first line of input contains the three integers W, v_{h}, and N, separated by spaces, with 1 <= W <= 10^{8}, 1 <= v_{h} <= 10^{6}, and 1 <= N <= 10^{5}.
The following N lines of input each contain two integers x_{i} and y_{i}, the horizontal and vertical positions respectively of the i^{th} left gate, with 1 <= x_{i}, y_{i} <= 10^{8}.
The next line of input contains an integer S, the number of skis, with 1 <= S <= 10^{6}.
The following S lines of input each contain one integer s_{j}, the speed of the j^{th} pair of skis, with 1 <= s_{j} <= 10^{6}.  Output
 If it is impossible to complete the race with any pair of skis, print the line IMPOSSIBLE. Otherwise, print the vertical speed s_{j} of the pair of skis that allows you to get through the race course in the shortest time.
 sample input

Case #1: 3 2 3 1 1 5 2 1 3 3 3 2 1 Case #2: 3 2 3 1 1 5 2 1 3 1 3
 sample output

Case #1: 2 Case #2: IMPOSSIBLE
 hint
 source
 waterloo 13 June, 2009