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 A44 {
    class Program {
        static void Main(string[] args) {

            int n = int.Parse(Console.ReadLine());
            int k = int.Parse(Console.ReadLine());
            int x = int.Parse(Console.ReadLine());
            int y = int.Parse(Console.ReadLine());
            var result = Method(k, x, y, n);
            Console.WriteLine(result);
        }

        private static int Method(int k, int x, int y, int n) {

            var result = 0;

            if (n <= k) {//k泊以内
                result += x * n;//k泊以内の宿泊料金×宿泊日数
            } else {//k泊以上
                result += x * k;
                result += (n - k) * y;//k泊以上の宿泊料金×宿泊日数
            }
            return result;
        }
    }
}