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题意翻译

Ann需要搭n次地铁。她有两种票可供选择：第一种票a卢布一张，可以搭一次地铁；第二种票b卢布一张，可以搭m次地铁。问：Ann最少需要多少卢布才能搭n次地铁？ 输入：一行四个整数：n,m,a,b。 输出：一行一个整数：Ann搭n次地铁最少需要的钱数（以卢布为单位） 输入输出样例1解释：买六张第一种票或买三张第二种票，等等（译者注：都需要6卢布）。

题目描述

Ann has recently started commuting by subway. We know that a one ride subway ticket costs $ a $ rubles. Besides, Ann found out that she can buy a special ticket for $ m $ rides (she can buy it several times). It costs $ b $ rubles. Ann did the math; she will need to use subway $ n $ times. Help Ann, tell her what is the minimum sum of money she will have to spend to make $ n $ rides?

输入输出格式

输入格式

The single line contains four space-separated integers $ n $ , $ m $ , $ a $ , $ b $ ( $ 1<=n,m,a,b<=1000 $ ) — the number of rides Ann has planned, the number of rides covered by the $ m $ ride ticket, the price of a one ride ticket and the price of an $ m $ ride ticket.

输出格式

Print a single integer — the minimum sum in rubles that Ann will need to spend.

输入输出样例

输入样例 #1

6 2 1 2

输出样例 #1

6

输入样例 #2

5 2 2 3

输出样例 #2

8

说明

In the first sample one of the optimal solutions is: each time buy a one ride ticket. There are other optimal solutions. For example, buy three $ m $ ride tickets.