Problem:
Score : 700 points

Problem Statement

N problems are proposed for an upcoming contest. Problem i has an initial integer score of A_i points.

M judges are about to vote for problems they like. Each judge will choose exactly V problems, independently from the other judges,
and increase the score of each chosen problem by 1.

After all M judges cast their vote, the problems will be sorted in non-increasing order of score, and the first P problems will be chosen for the problemset.
Problems with the same score can be ordered arbitrarily, this order is decided by the chief judge.

How many problems out of the given N have a chance to be chosen for the problemset?

Constraints

2 \le N \le 10^5

1 \le M \le 10^9

1 \le V \le N - 1

1 \le P \le N - 1

0 \le A_i \le 10^9

Input

Input is given from Standard Input in the following format:

N M V P
A_1 A_2 ... A_N

Output

Print the number of problems that have a chance to be chosen for the problemset.

Sample Input 1

6 1 2 2
2 1 1 3 0 2

Sample Output 1

5

If the only judge votes for problems 2 and 5, the scores will be 2 2 1 3 1 2.
The problemset will consist of problem 4 and one of problems 1, 2, or 6.

If the only judge votes for problems 3 and 4, the scores will be 2 1 2 4 0 2.
The problemset will consist of problem 4 and one of problems 1, 3, or 6.

Thus, problems 1, 2, 3, 4, and 6 have a chance to be chosen for the problemset. On the contrary, there is no way for problem 5 to be chosen.

Sample Input 2

6 1 5 2
2 1 1 3 0 2

Sample Output 2

3

Only problems 1, 4, and 6 have a chance to be chosen.

Sample Input 3

10 4 8 5
7 2 3 6 1 6 5 4 6 5

Sample Output 3

8

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Linq;

public class Program
{
    private int N, M, V, P;
    private int[] A;

    public void Solve()
    {
        var sc = new Scanner();
        N = sc.NextInt();
        M = sc.NextInt();
        V = sc.NextInt();
        P = sc.NextInt();
        A = sc.IntArray();
        Array.Sort(A, (a, b) => -a.CompareTo(b));
        /*
         * target強がP未満
         */
        int ok = 0;
        int ng = N;
        while (ng - ok > 1)
        {
            int med = (ok + ng) / 2;

            if (C(med))
            {
                ok = med;
            }
            else
            {
                ng = med;
            }
        }

        Console.WriteLine(ng);
    }

    bool C(int n)
    {
        int[] cp = A.ToArray();
        long tmp = (long) M * V;
        int[] t = new int[N];
        for (int i = 0; i < N; i++)
        {
            t[i] = M;
        }

        int target = A[n] + M;
        cp[n] += M;
        tmp -= M;
        t[n] -= M;

        for (int i = N - 1; i >= 0; i--)
        {
            if (cp[i] < target)
            {
                int d = Math.Min(M, target - cp[i]);
                t[i] -= d;
                cp[i] += d;
                tmp -= d;
            }
        }
        // 全部 target 以上


        for (int i = 0; i < N; i++)
        {
            if (cp[i] > target)
            {
                int d = t[i];
                cp[i] += d;
                t[i] -= d;
                tmp -= d;
            }
        }

        var index = new int[N];
        for (int i = 0; i < N; i++)
        {
            index[i] = i;
        }

        Array.Sort(index, (a, b) => -t[a].CompareTo(t[b]));

        for (int i = 0; i < N && tmp > 0; i++)
        {
            int d = t[index[i]];
            cp[index[i]] += d;
            t[index[i]] -= d;
            tmp -= d;
        }

        if (tmp > 0) return false;

        return cp.Count(i => i > target) < P;
    }

    public static void Main(string[] args)
    {
        new Program().Solve();
    }
}

class Scanner
{
    public Scanner()
    {
        _pos = 0;
        _line = new string[0];
    }

    const char Separator = ' ';
    private int _pos;
    private string[] _line;

    #region スペース区切りで取得

    public string Next()
    {
        if (_pos >= _line.Length)
        {
            _line = Console.ReadLine().Split(Separator);
            _pos = 0;
        }

        return _line[_pos++];
    }

    public int NextInt()
    {
        return int.Parse(Next());
    }

    public long NextLong()
    {
        return long.Parse(Next());
    }

    public double NextDouble()
    {
        return double.Parse(Next());
    }

    #endregion

    #region 型変換

    private int[] ToIntArray(string[] array)
    {
        var result = new int[array.Length];
        for (int i = 0; i < array.Length; i++)
        {
            result[i] = int.Parse(array[i]);
        }

        return result;
    }

    private long[] ToLongArray(string[] array)
    {
        var result = new long[array.Length];
        for (int i = 0; i < array.Length; i++)
        {
            result[i] = long.Parse(array[i]);
        }

        return result;
    }

    private double[] ToDoubleArray(string[] array)
    {
        var result = new double[array.Length];
        for (int i = 0; i < array.Length; i++)
        {
            result[i] = double.Parse(array[i]);
        }

        return result;
    }

    #endregion

    #region 配列取得

    public string[] Array()
    {
        if (_pos >= _line.Length)
            _line = Console.ReadLine().Split(Separator);

        _pos = _line.Length;
        return _line;
    }

    public int[] IntArray()
    {
        return ToIntArray(Array());
    }

    public long[] LongArray()
    {
        return ToLongArray(Array());
    }

    public double[] DoubleArray()
    {
        return ToDoubleArray(Array());
    }

    #endregion
}