Problem B. Avengers and Shawarma

Author:A. Usmanov. Translation: V. Toropov.   Time limit:1 sec
Input file:Standard input   Memory limit:256 Mb
Output file:Standard output  

Statement

Avengers defeated alien invaders and decided to eat some shawarma.

Hulk ordered N shawarmas. M shawarmen immediately started to cook his order. Each shawarman can cook his first shawarma in T minutes. Each next shawarma requires S minutes more to cook than the previous one.

HULK SMASHes!!! awaiting his shawarmas.

You should help Avengers to find out the time when all shawarmas ordered by Hulk will be ready and green giant will calm down. Of course, Jarvis could solve this task, but Iron Man's suit is broken and only air conditioner is working right now.

Input format

First line contains two integers N and M — number of shawarmas ordered by Hulk, and number of shawarmen who are going to complete his order.

Second line contains two integers T and S — time needed to cook the first shawarma and the difference of cooking time between two shawarmas in the row.

Output format

Print one integer — time needed to complete Hulk's order.

Constraints

1 ≤ N, M, T ≤ 100

0 ≤ S ≤ 100

Sample tests

No. Standard input Standard output
1
5 2
10 5
45
2
13 4
4 1
22
3
10 1
5 0
50

0.148s 0.018s 13