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;

namespace Vanila
{
    public 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());
            if (N > K)
            {
                Console.WriteLine((N - K) * Y + K * X);
            }
            else {
                Console.WriteLine(N * X);
            }
         }
        public static long Max(long a, long b)
        {
            if (a <= b) return b;
            else return a;
        }
        public static long Min(long a, long b)
        {
            if (a == Max(a, b)) return b;
            else return a;
        }
        public static long GCD(long a, long b)
        {
            while (Max(a, b) % Min(a, b) != 0)
            {
                if (a < b) b %= a;
                else a %= b;
            }
            return Min(a, b);
        }
        public static long LCM(long a, long b)
        {
            return a * b / GCD(a, b);
        }
    }
}