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;

namespace AtCoder.ABC044
{
    class ProblemA
    {
        static void Main()
        {
            var n = ReadInt();
            var k = ReadInt();
            var x = ReadInt();
            var y = ReadInt();
            Console.WriteLine($"{Math.Min(n, k) * x + Math.Max(0, n - k) * y }");
        }

        public static string ReadLine() => Console.ReadLine();

        public static int ReadInt() => int.Parse(ReadLine());

        public static int[] ReadIntArray() => ReadLine().Split(' ').Select(v => int.Parse(v)).ToArray();
    }
}