B'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 <= t <= 10 ,000 ). Description of the test cases follows. The first line of each test case contains two integers a and b (1 <= q a, b <= q 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 <= q n, m <= q 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. '...