[Solution] Buy Lamps CodeChef Solution

An electronics shop sells red and blue lamps. A red lamp costs X$X$ rupees and a blue lamp costs Y$Y$ rupees.

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

Input Format

• The first line of input will contain a single integer T$T$, denoting the number of test cases.

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• Each test case consists of a single line containing four space-separated integers N,K,X,Y$N,K,X,Y$.

Output Format

For each test case, output on a new line the minimum amount of money Chef needs to pay in order to

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 buy N$N$ lamps such that at least K$K$ of the lamps bought are red.

Constraints

• 1≤T≤103$1\le T\le {10}^3$
• 1≤N≤108$1\le N\le {10}^8$
• 0≤K≤N$0\le K\le N$
• 1≤X,Y≤10$1\le X,Y\le 10$