Problem: There is a hotel with the following accommodation fee:

* X yen (the currency of Japan) per night, for the first K nights
* Y yen per night, for the (K+1)-th and subsequent nights



Tak is staying at this hotel for N consecutive nights. Find his total accommodation fee.

Constraints

* 1 \leq N, K \leq 10000
* 1 \leq Y < X \leq 10000
* N,\,K,\,X,\,Y are integers.

Input

The input is given from Standard Input in the following format:


N
K
X
Y


Output

Print Tak's total accommodation fee.

Examples

Input

5
3
10000
9000


Output

48000


Input

2
3
10000
9000


Output

20000

Solution (in CPP):

Solution:
#include <cstdio>
int main()
{
	int a,b,c,d;
scanf("%d %d %d %d",&a,&b,&c,&d);
if(a<=b) 
{
	printf("%d\n",a*c);
}
else 
{
	printf("%d\n",b*c+(a-b)*d);
}
return 0;
}