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<bits/stdc++.h>
using namespace std;
int main(){
    int N,K,X,Y;
    cin>>N>>K>>X>>Y;
    if(K>=N)cout<<X*N<<endl;
    else cout<<X*K+Y*(N-K)<<endl;
}