A. Cheap Travel

time limit per test

1 second

memory limit per test

256 megabytes

input

standard input

output

standard output

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?

Input

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.

Output

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

Sample test(s)

input

6 2 1 2

output

6

input

5 2 2 3

output

8

Note

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.

解题说明：此题是一道简单的模拟题，要么花少点钱每次买票，要么花多一点的钱买一次票用m次，简单比较即可。

#include<iostream>#include<cstdio>#include<algorithm>#include<cstring>#include<string>#include<cmath>using namespace std;int main(){int n,m,a,b,sum,med;sum=0;scanf("%d%d%d%d",&n,&m,&a,&b);if(m*a<b){med=m*a;}else{med=b;}sum=sum+(n/m)*med;n=n%m;if(n>0){if((n*a)<b){sum=sum+n*a;}else{sum=sum+b;}}printf("%d\n",sum);return 0;}