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.Linq;

namespace easy_Atcoder
{
    class Program
    {
        static void Main(string[] args)
        {
            // float[] ABN = Array.ConvertAll(Console.ReadLine().Split(),float.Parse);
            // int[] nx = Array.ConvertAll(Console.ReadLine().Split(),int.Parse);
            // long K = long.Parse(Console.ReadLine());
            // int[] ABC = Array.ConvertAll(Console.ReadLine().Split(),int.Parse);
            // string S = Console.ReadLine();
            int N = int.Parse(Console.ReadLine());
            int K = int.Parse(Console.ReadLine());
            int X = int.Parse(Console.ReadLine());
            int Y = int.Parse(Console.ReadLine());
            // string A = Console.ReadLine();
            // string B = Console.ReadLine();
            // var s = (new string[] { Console.ReadLine(), Console.ReadLine(), Console.ReadLine() }).Select(int.Parse);
            // int[] abc = Array.ConvertAll(Console.ReadLine().Split(),int.Parse);
            // var X = Console.ReadLine();
            // int[,] abcd = new int[NMQ[2],4];
            // int[] A = new int[NMQ[0]];
            // int[] A_num = new int[NMQ[0]];
            // for(int i = 0; i<A_num.Length;i++){
            //     A_num[i] = 0;
            // }
            int fee =0;
            for(int i = 1; i <=N ;i++){
                if(i <= K){
                    fee += X;
                }else{
                    fee += Y;
                }
            }
            Console.WriteLine(fee);
        }
        //最小公倍数
        static long Lcm(long a, long b)
        {
            return a/Gcd(a,b)*b;
        }

        //最大公約数
        static long Gcd(long m, long n)
        {
            if(n==0)return m;
            return Gcd(n,m%n);
        }
    }
}