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# (.NET Core 3.1.201) (judge verdict: Wrong Answer)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace cont1
{
    class Program
    {
        static void Main(string[] args)
        {
            int n, k, x, y ,result=0;
            Console.WriteLine("Enter N (the nomber of nights)");
            n = int.Parse(Console.ReadLine());

            Console.WriteLine("Enter K (the nomber of firist nights)");
            k = int.Parse(Console.ReadLine());

            Console.WriteLine("Enter X (the cost of firist nights)");
            x = int.Parse(Console.ReadLine());

            Console.WriteLine("Enter Y (the cost of scond nights)");
            y = int.Parse(Console.ReadLine());

            for(int i=0;i<k ;i++)
            {
                result += x;
            }

            for (int i =k ; i < n; i++)
            {
                result += y;
            }

            Console.WriteLine(result);
        }
    }
}