Souvenir

Problem Description

Today is the 1st anniversary of BestCoder. Soda, the contest manager, wants to buy a souvenir for each contestant. You can buy the souvenir one by one or set by set in the shop. The price for a souvenir is
p
yuan and the price for a set of souvenirs if
q
yuan. There's m
souvenirs in one set.There's
n
contestants in the contest today. Soda wants to know the minimum cost needed to buy a souvenir for each contestant.
Input

There are multiple test cases. The first line of input contains an integer
T
(1≤T≤105),
indicating the number of test cases. For each test case:There's a line containing 4 integers
n,m,p,q
(1≤n,m,p,q≤104).
Output

For each test case, output the minimum cost needed.
Sample Input

2
1 2 2 1
1 2 3 4

Sample Output

1
3