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: Runtime Error)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    class Cards
    {
        readonly int N = 51;

        long[][] dp;
        int[] cards;

        int n, target;

        public Cards(int n, int a)
        {
            dp = new long[N][];

            for (int i = 0; i < N; i++)
            {
                dp[i] = Enumerable.Repeat((long)-1, N * N).ToArray();
            }

            cards = new int[N];

            this.n = n;
            target = a;
        }

        public void MakeMap(string[] source)
        {
            for (int i = 0; i < source.Length; i++)
            {
                cards[i + 1] = int.Parse(source[i]);
            }
        }

        public long Solve()
        {
            dp[0][0] = 1;

            int sum = 0;

            for (int k = 1; k <= n; k++)
            {
                sum += cards[k];

                for (int i = k; i > 0; i--)
                {
                    for (int j = sum; j >= cards[k]; j--)
                    {
                        if (dp[i - 1][j - cards[k]] == -1) continue;

                        if (dp[i][j] == -1) dp[i][j] = 0;

                        dp[i][j] += dp[i - 1][j - cards[k]];
                    }
                }
            }

            long ret = 0;

            for (int i = 1; i <= n; i++)
            {
                if (dp[i][i * target] != -1) ret += dp[i][i * target];
            }
            return ret;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            string[] input = Console.ReadLine().Split(' ');
            int n = int.Parse(input[0]);
            int a = int.Parse(input[1]);

            Cards cs = new Cards(n, a);

            input = Console.ReadLine().Split(' ');

            cs.MakeMap(input);

            Console.WriteLine(cs.Solve());
        }
    }
}