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.Generic;
using System.Linq;
using System.Text;
using System.IO;

class Program
{

    static void Main()
    {
        var sc = new Scanner();
        int n = sc.Int();
        int k = sc.Int();
        int x = sc.Int();
        int y = sc.Int();
        if (n<=k)
        {
            Console.WriteLine(n*x);
        }
        else
        {
            Console.WriteLine(k*x+(n-k)*y);
        }
    }
}
static class Extentions
{
    public static IEnumerable<T> Outpuut<T>(this IEnumerable<T> source)
    {
        foreach (var item in source)
        {
            Console.WriteLine(item);
        }
        return source;
    }
    public static T Print<T>(this T obj)
    {
        Console.WriteLine(obj);
        return obj;
    }
}
public class Scanner
{
    public Scanner()
    {
        byte[] inputBuffer = new byte[1024];
        System.IO.Stream inputStream = Console.OpenStandardInput(inputBuffer.Length);
        Console.SetIn(new System.IO.StreamReader(inputStream, Console.InputEncoding, false, inputBuffer.Length));
    }
    string[] buffer = new string[0];
    int index = 0;
    public char[] SplitChars { get; set; }
    public string String()
    {
        if (index < buffer.Length) return buffer[index++];
        string line = "";
        while (line == "") line = Console.ReadLine();
        if (line == null) return null;
        buffer = line.Split(SplitChars, StringSplitOptions.RemoveEmptyEntries);
        index = 0;
        return buffer[index++];
    }
    public string[] StringArray(int count) { return NextArray(count, x => x); }
    public void Skip(int count) { for (int i = 0; i < count; i++) String(); }
    public T Next<T>(Func<string, T> parser) { return parser(String()); }
    public T[] NextArray<T>(int count, Func<string, T> parser)
    {
        var arr = new T[count];
        for (int i = 0; i < count; i++) arr[i] = Next(parser);
        return arr;
    }
    public int Int() { return Next(int.Parse); }
    public int[] IntArray(int count) { return NextArray(count, int.Parse); }
    public double Double() { return Next(double.Parse); }
    public double[] DoubleArray(int count) { return NextArray(count, double.Parse); }
}