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 static System.Console;
using static System.Math;
using static System.Array;
using static Read;

class Program
{
    public static void Main()
    {
        var n = Int();
        var k = Int();
        var x = Int();
        var y = Int();
        int a, b;
        if(k < n)
        {
            a = k;
            b = n - k;
        }
        else
        {
            a = n;
            b = 0;
        }
        WriteLine($"{a * x + b * y}");
    }
}

static class Read
{
    static Queue<string> queue = new Queue<string>();
    static void Enqueue()
    {
        ReadLine().Split(' ').ToList().ForEach(a => queue.Enqueue(a));
    }
    public static int Int()
    {
        if(queue.Count == 0)
            Enqueue();
        return int.Parse(queue.Dequeue());
    }
    public static long Long()
    {
        if(queue.Count == 0)
            Enqueue();
        return long.Parse(queue.Dequeue());
    }
    public static double Double()
    {
        if(queue.Count == 0)
            Enqueue();
        return double.Parse(queue.Dequeue());
    }
    public static string String()
    {
        if(queue.Count == 0)
            Enqueue();
        return queue.Dequeue();
    }
    public static int[] IntArray()
    {
        return ReadLine().Split(' ').Select(a => int.Parse(a)).ToArray();
    }
    public static long[] LongArray()
    {
        return ReadLine().Split(' ').Select(a => long.Parse(a)).ToArray();
    }
    public static double[] DoubleArray()
    {
        return ReadLine().Split(' ').Select(a => double.Parse(a)).ToArray();
    }
    public static string[] StringArray()
    {
        return ReadLine().Split(' ');
    }
}