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

public class Don
{
    public Don() { }
    private Reader r = new Reader();
    private static List<int> A;

    public void Solve()
    {
        var N = r.nextInt();
        var K = r.nextInt();
        var X = r.nextInt();
        var Y = r.nextInt();

        var ret = 0;
        if (N > K)
        {
            ret = X * K + Y * (N - K);
        } else
        {
            ret = X * N;
        }

        Console.WriteLine(ret);
    }

    public static void Main(string[] args)
    {
        new Don().Solve();
    }

}

class Reader
{
    string[] s;
    int i;
    char[] cs = new char[] { ' ' };

    public Reader()
    {
        s = new string[0];
        i = 0;
    }

    public string next()
    {
        if (i < s.Length)
            return s[i++];
        var input = Console.ReadLine();
        while (input == "")
            input = Console.ReadLine();
        s = input.Split(cs, StringSplitOptions.RemoveEmptyEntries);
        i = 0;
        return next();
    }

    public int nextInt()
    {
        return int.Parse(next());
    }

    public long nextLong()
    {
        return long.Parse(next());
    }

    public double nextDouble()
    {
        return double.Parse(next());
    }

}