CF1793A Yet Another Promotion

The famous store "Second Food" sells groceries only two days a month. And the prices in each of days differ. You wanted to buy $ n $ kilos of potatoes for a month. You know that on the first day of the month $ 1 $ kilo of potatoes costs $ a $ coins, and on the second day $ b $ coins. In "Second Food" you can buy any integer kilograms of potatoes. Fortunately, "Second Food" has announced a promotion for potatoes, which is valid only on the first day of the month — for each $ m $ kilos of potatoes you buy, you get $ 1 $ kilo as a gift! In other words, you can get $ m + 1 $ kilograms by paying for $ m $ kilograms. Find the minimum number of coins that you have to spend to buy at least $ n $ kilos of potatoes.

Each test contains multiple test cases. The first line contains the number of test cases $ t $ ( $ 1 \le t \le 10\,000 $ ). Description of the test cases follows. The first line of each test case contains two integers $ a $ and $ b $ $ (1 \leq a, b \leq 10^9) $ — the prices of $ 1 $ kilo of potatoes on the first and second days, respectively. The second line contains two integers $ n $ and $ m $ $ (1 \leq n, m \leq 10^9) $ — the required amount of potatoes to buy and the amount of potatoes to use the promotion.

For each test case print one integer — the minimum number of coins that you have to pay to buy at least $ n $ kilos of potatoes.

In the first test case, on the first day you buy $ 1 $ kilo and get $ 1 $ more for a promotion. On the second day, you can buy $ 1 $ kilo of potatoes. Thus, you will spend $ 5+4=9 $ coins in total. In the second test case, on the first day you buy $ 2 $ kilo and get another $ 1 $ more for a promotion. This way you will spend $ 2 \cdot 5 = 10 $ coins.