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# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections;
using System.Collections.Generic;
using System.Text;
using System.Linq;
using static MyIO;


public class A
{
    public static void Main()
    {
        int n = GetInt();
        int k = GetInt();
        int x = GetInt();
        int y = GetInt();
        int ans = 0;
        for (int i = 1; i <= n; i++)
        {
            if (k < i)
            {
                ans += y;
            }
            else
            {
                ans += x;
            }
        }
        Console.WriteLine(ans);
    }
}

public static class MyIO
{
    private static string[] args = null;
    private static int num = -1;
    private static int used = -1;

    private static string getArg()
    {
        if (used == num)
        {
            args = Console.ReadLine().Split(' ');
            num = args.Length;
            used = 0;
        }
        return args[used++];
    }

    public static int GetInt() { return int.Parse(getArg()); }
    public static long GetLong() { return long.Parse(getArg()); }
    public static double GetDouble() { return double.Parse(getArg()); }
    public static string GetString() { return getArg(); }
}