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.

著名超市“第二食品”每月仅营业两天，且这两天的价格各不相同。你希望在一个月内购买 $n$ 千克土豆。已知该月第一天 1 千克土豆售价为 $a$ 枚金币，第二天则为 $b$ 枚金币。“第二食品”允许你购买任意整数千克的土豆。

幸运的是，“第二食品”针对土豆推出了一项仅在该月第一天有效的促销活动：每购买 $m$ 千克土豆，即可获赠 1 千克！换言之，你只需支付 $m$ 千克的钱，便可获得 $m + 1$ 千克土豆。

求购买至少 $n$ 千克土豆所需的最少金币数。

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.

每个测试包含多个测试用例。第一行包含测试用例的数量 $t$（$1 \le t \le 10\,000$）。随后是各测试用例的描述。

每个测试用例的第一行包含两个整数 $a$ 和 $b$（$1 \leq a, b \leq 10^9$）——分别为第一天和第二天每千克土豆的价格。

每个测试用例的第二行包含两个整数 $n$ 和 $m$（$1 \leq n, m \leq 10^9$）——分别为需要购买的土豆总量以及使用促销所需达到的土豆数量。

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.

对于每个测试用例，输出一个整数——购买至少 $n$ 千克土豆所需的最少硬币数。

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.

在第一个测试用例中，第一天你购买 $1$ 千克，并因促销活动额外获得 $1$ 千克；第二天你可以购买 $1$ 千克土豆。因此，你总共将花费 $5+4=9$ 枚金币。

在第二个测试用例中，第一天你购买 $2$ 千克，并因促销活动额外获得 $1$ 千克。这样你将花费 $2 \cdot 5 = 10$ 枚金币。