A812 | Rest Stops--Silver

Farmer John and his personal trainer Bessie are hiking up Mount Vancowver. For
their purposes (and yours), the mountain can be represented as a long straight
trail of length $L$ meters ($1 \leq L \leq 10^6$). Farmer John will hike the
trail at a constant travel rate of $r_F$ seconds per meter ($1 \leq r_F \leq
10^6$). Since he is working on his stamina, he will not take any rest stops
along the way.
Bessie, however, is allowed to take rest stops, where she might find some
tasty grass. Of course, she cannot stop just anywhere! There are $N$ rest
stops along the trail ($1 \leq N \leq 10^5$); the $i$-th stop is $x_i$ meters
from the start of the trail ($0 < x_i < L$) and has a tastiness value $c_i$
($1 \leq c_i \leq 10^6$). If Bessie rests at stop $i$ for $t$ seconds, she
receives $c_i \cdot t$ tastiness units.
When not at a rest stop, Bessie will be hiking at a fixed travel rate of $r_B$
seconds per meter ($1 \leq r_B \leq 10^6$). Since Bessie is young and fit,
$r_B$ is strictly less than $r_F$.
Bessie would like to maximize her consumption of tasty grass. But she is
worried about Farmer John; she thinks that if at any point along the hike she
is behind Farmer John on the trail, he might lose all motivation to continue!
Help Bessie find the maximum total tastiness units she can obtain while making
sure that Farmer John completes the hike.

The first line of input contains four integers: $L$, $N$, $r_F$, and $r_B$.
The next $N$ lines describe the rest stops. For each $i$ between $1$ and $N$,
the $i+1$-st line contains two integers $x_i$ and $c_i$, describing the
position of the $i$-th rest stop and the tastiness of the grass there.
It is guaranteed that $r_F > r_B$, and $0 < x_1 < \dots < x_N < L $. **Note
that $r_F$ and $r_B$ are given in seconds per meter!**

A single integer: the maximum total tastiness units Bessie can obtain.