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.util.*;
import java.io.*;

class Main {

	void solve() {

		int n = inint(), k = inint(), x = inint(), y = inint();
		
		if (n <= k) out.println(n * x);
		else {
			out.println(k * x + (n - k) * y);
		}

	}

	public static Scanner in = new Scanner(System.in);
	public static PrintWriter out = new PrintWriter(System.out);

	public static void main(String[] args) {
		new Main().solve();
		out.flush();
	}

	public int inint() {
		return in.nextInt();
	}

	public long inlong() {
		return in.nextLong();
	}

	public String instr() {
		return in.next();
	}

	public char inchar() {
		return in.next().charAt(0);
	}

	public int[] inintar(int num) {
		int[] a = new int[num];
		for (int i=0; i<num; i++) {
			a[i] = in.nextInt();
		}
		return a;
	}

	public char[] incharar() {
		return in.next().toCharArray();
	}

	public char[][] incharmap(int h, int w) {
		char[][] a = new char[h][w];
		for (int i=0; i<h; i++) {
			String temp = in.next();
			for (int j=0; j<w; j++) {
				a[i][j] = temp.charAt(j);
			}
		}
		return a;
	}

}