Solve the programming task below in a Python markdown code block.
An electronics shop sells red and blue lamps. A red lamp costs X rupees and a blue lamp costs Y rupees.

Chef is going to buy exactly N lamps from this shop. Find the minimum amount of money Chef needs to pay such that at least K of the lamps bought are red.

------ Input Format ------ 

- The first line of input will contain a single integer T, denoting the number of test cases.
- Each test case consists of a single line containing four space-separated integers N, K, X, Y.

------ Output Format ------ 

For each test case, output on a new line the minimum amount of money Chef needs to pay in order to buy N lamps such that at least K of the lamps bought are red.

------ Constraints ------ 

$1 ≤ T ≤ 10^{3}$
$1 ≤ N ≤10^{8}$
$0 ≤ K ≤ N$
$1 ≤ X, Y ≤10$

----- Sample Input 1 ------ 
4
2 2 5 1
4 1 3 1
3 0 4 7
5 2 3 4

----- Sample Output 1 ------ 
10
6
12
15

----- explanation 1 ------ 
Test case $1$: Chef buys $2$ red lamps with $2 \cdot 5 = 10$ rupees.

Test case $2$: Chef buys $1$ red lamp and $3$ blue lamps with $1 \cdot 3 + 3\cdot 1 = 6$ rupees.

Test case $3$: Chef buys $3$ red lamps with $3 \cdot 4 = 12$ rupees.