## Algorithm

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 nmab (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.

Examples
input
Copy
`6 2 1 2`
output
Copy
`6`
input
Copy
`5 2 2 3`
output
Copy
`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.

## Code Examples

### #1 Code Example with C++ Programming

```Code - C++ Programming```

``````#include <stdio.h>

using namespace std;

int main() {
int n, m, a, b, res = 1e9;
scanf("%d%d%d%d", &n, &m, &a, &b);

for(int i = 0; i <= 1000; i++)
if(i * m + j >= n)
res = min(res, i * b + max(0, n - i * m) * a);

printf("%d\n", res);

return 0;
}``````
Copy The Code &

Input

cmd
6 2 1 2

Output

cmd
6