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.Threading.Tasks;

namespace ABC044A___高橋君とホテルイージー
{
    class Program
    {
        static void Main(string[] args)
        {
            string input1 = Console.ReadLine();
            string input2 = Console.ReadLine();
            string input3 = Console.ReadLine();
            string input4 = Console.ReadLine();

            int numberOfStay = int.Parse(input1);
            int boundNumberOfStay = int.Parse(input2);
            int firstHalfPrice = int.Parse(input3);
            int secondtHalfPrice = int.Parse(input4);
            int totalPrice = 0;

            bool isOverBoundNum = (numberOfStay - boundNumberOfStay > 0);
            if (isOverBoundNum)
            {
                totalPrice = firstHalfPrice * boundNumberOfStay + secondtHalfPrice * (numberOfStay - boundNumberOfStay);
            }
            else
            {
                totalPrice = firstHalfPrice * numberOfStay;
            }

           

            Console.WriteLine(totalPrice.ToString());
        }
    }
}