Problem K

Nora the kitesurfer is taking part in a race across the Frisian islands, a very long and thin archipelago in the north of the Netherlands. The race takes place on the water and follows a straight line from start to finish. Any islands on the route must be jumped over – it is not allowed to surf around them.

The length of the race is $s$ metres and the archipelago consists of a number of non-intersecting intervals between start and finish line. During the race, Nora can move in two different ways:

  1. Nora can surf between any two points at a speed of $1$ metre per second, provided there are no islands between them.

  2. Nora can jump between any two points if they are at most $d$ metres apart and neither of them is on an island. A jump always takes $t$ seconds, regardless of distance covered.

While it is not possible to land on or surf across the islands, it is still allowed to visit the end points of any island.

\includegraphics[width=0.8\textwidth ]{samples}
Figure 1: Illustration of the two sample cases.

Your task is to find the shortest possible time Nora can complete the race in. You may assume that no island is more than $d$ metres long. In other words it is always possible to finish the race.


The input consists of:

  • One line with three integers $s,d$ and $t$ ($1 \le s,d,t \le 10^9$), where $s$ is the length of the race in metres, $d$ is the maximal jump distance in metres, and $t$ is the time needed for each jump in seconds.

  • One line with an integer $n$ ($0 \le n \le 500$), the number of islands.

  • $n$ lines, the $i$th of which contains two integers $\ell _ i$ and $r_ i$ ($0 < \ell _ i < r_ i < s$ and $r_ i-\ell _ i \le d$), giving the boundaries of the $i$th island in metres, relative to the starting point.

The islands do not touch and are given from left to right, that is $r_ i < \ell _{i+1}$ for each valid $i$.


Output one number, the shortest possible time in seconds needed to complete the race. It can be shown that this number is always an integer.

Sample Input 1 Sample Output 1
9 3 4
2 4
7 8
Sample Input 2 Sample Output 2
12 5 3
1 3
5 7
8 11

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