Problem:
Score : 100 points

Problem Statement

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.

Sample Input 1

5
3
10000
9000

Sample Output 1

48000

The accommodation fee is as follows:

10000 yen for the 1-st night

10000 yen for the 2-nd night

10000 yen for the 3-rd night

9000 yen for the 4-th night

9000 yen for the 5-th night

Thus, the total is 48000 yen.

Sample Input 2

2
3
10000
9000

Sample Output 2

20000

Solution in C++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <bits/stdc++.h>
using namespace std;

#define ll long long
#define p_q priority_queue
#define rep(i,n) for(int (i)=0;(i)<(n);(i)++)
#define str string
#define s_vec(data,name,vertical,horizontal) vector<vector<(data)>> (name)((vertical),vector<(data)>((horizontal)));


int main(){
    int n,k,x,y;

    cin>>n>>k>>x>>y;

    int ans;
    if(k<n){
       ans = k*x + (n-k)*y;
    }else{
        ans = n*x;
    }
    
    cout<<ans;

}