CF466A.Cheap Travel

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?

安娜最近开始乘坐地铁通勤。已知单程地铁票的价格为 $a$ 卢布。此外，安娜发现她可以购买一种特殊车票，每张可乘坐 $m$ 次（她可以购买多张），每张售价为 $b$ 卢布。安娜已计算出自己总共需要乘坐地铁 $n$ 次。请帮助安娜计算：她完成这 $n$ 次乘车所需的最少花费是多少？

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.

单行包含四个以空格分隔的整数 $n$、$m$、$a$、$b$（$1 ≤ n, m, a, b ≤ 1000$）——分别表示安计划乘坐的次数、$m$ 次票所覆盖的乘坐次数、单次票的价格以及 $m$ 次票的价格。

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

输出一个整数——Ann 需要花费的最少金额（单位：卢布）。

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.

在第一个样例中，一种最优解是：每次购买一张单程票。也存在其他最优解，例如购买三张 $m$ 次票。