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;

public class Program
{
    public static long loop;

    static long n;
    static long k;
    static long x;
    static long y;

    static string output;

    public static void Main(string[] args)
    {
        loop = 0;

        Input();
        Proc();
        Output();
    }

    static void Input()
    {
        string[] input = null;

        // スペース区切りの整数の入力(横、個別)
        input = Console.ReadLine().Split(' ');

        n = long.Parse(input[0]);   // 回数

        // スペース区切りの整数の入力(横、個別)
        input = Console.ReadLine().Split(' ');

        k = long.Parse(input[0]);   // 閾値

        // スペース区切りの整数の入力(横、個別)
        input = Console.ReadLine().Split(' ');

        x = long.Parse(input[0]);   // 前

        // スペース区切りの整数の入力(横、個別)
        input = Console.ReadLine().Split(' ');

        y = long.Parse(input[0]);   // 後
    }

    static void Proc()
    {
        long money = 0;

        for (long i = 1; i <= n; i++)
        {
            if (i <= k)
            {
                money += x;
            }
            else
            {
                money += y;
            }
        }

        output = string.Format("{0}", money);
    }

    static void Output()
    {
        // 出力
        Console.WriteLine(output);
        output = string.Empty;
    }
}