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 Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.io.PrintWriter;
import java.util.Scanner;

public class Main {
    static private Scanner scan = new Scanner(System.in);
    public static void main(String args[]) {
        int int1 = Integer.parseInt(scan.next());
        int int2 = Integer.parseInt(scan.next());
        int int3 = Integer.parseInt(scan.next());
        int int4 = Integer.parseInt(scan.next());
//        String temp_str1 = "";
//        String temp_str2 = "";
        int temp_int1 = 0;
//        String str1 = scan.next();
//        String str2 = scan.next();
//        String str3 = scan.next();
        int ans_int = 0;
        String ans = "";
//        double ans_dob = 0;
        if(int2>=int1){
            ans_int = int1*int3;
        }else{
            ans_int = int2*int3 + (int1-int2)*int4;
        }

        PrintWriter out = new PrintWriter(System.out);
        out.println(ans_int);
        out.flush();
        scan.close();
    }
}