Problem: While Grisha was celebrating New Year with Ded Moroz, Misha gifted Sasha a small rectangular pond of size n × m, divided into cells of size 1 × 1, inhabited by tiny evil fishes (no more than one fish per cell, otherwise they'll strife!).

The gift bundle also includes a square scoop of size r × r, designed for fishing. If the lower-left corner of the scoop-net is located at cell (x, y), all fishes inside the square (x, y)...(x + r - 1, y + r - 1) get caught. Note that the scoop-net should lie completely inside the pond when used.

Unfortunately, Sasha is not that skilled in fishing and hence throws the scoop randomly. In order to not frustrate Sasha, Misha decided to release k fishes into the empty pond in such a way that the expected value of the number of caught fishes is as high as possible. Help Misha! In other words, put k fishes in the pond into distinct cells in such a way that when the scoop-net is placed into a random position among (n - r + 1)·(m - r + 1) possible positions, the average number of caught fishes is as high as possible.

Input

The only line contains four integers n, m, r, k (1 ≤ n, m ≤ 105, 1 ≤ r ≤ min(n, m), 1 ≤ k ≤ min(n·m, 105)).

Output

Print a single number — the maximum possible expected number of caught fishes.

You answer is considered correct, is its absolute or relative error does not exceed 10 - 9. Namely, let your answer be a, and the jury's answer be b. Your answer is considered correct, if <image>.

Examples

Input

3 3 2 3


Output

2.0000000000


Input

12 17 9 40


Output

32.8333333333

Note

In the first example you can put the fishes in cells (2, 1), (2, 2), (2, 3). In this case, for any of four possible positions of the scoop-net (highlighted with light green), the number of fishes inside is equal to two, and so is the expected value.

<image>

Solution (in JAVA):

Solution:
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Comparator;
import java.util.HashSet;
import java.util.PriorityQueue;


public class ProblemD {
  static class Pair{
    int row, column;
    Pair(int row, int column){
      this.row = row;
      this.column = column;
    }

    @Override
    public boolean equals(Object obj) {
      if(!(obj instanceof Pair)){
        return false;
      }
      Pair that = (Pair) obj;
      return this.row == that.row && this.column == that.column;
    }

    @Override
    public int hashCode(){
      return this.row * 10007 + this.column;
    }
  }
  static class PQObject{
    long value;
    int row, column;
    PQObject(long value, int row, int column){
      this.value = value;
      this.row = row;
      this.column = column;
    }
  }

  static class PQComparator implements Comparator<PQObject> {
    @Override
    public int compare(PQObject o1, PQObject o2){
      return (int) (o2.value - o1.value);
    }
  }

  public static void main(String [] args) throws IOException {
    BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
    String [] line = reader.readLine().split(" ");
    int n = Integer.parseInt(line[0]);
    int m = Integer.parseInt(line[1]);
    int r = Integer.parseInt(line[2]);
    int k = Integer.parseInt(line[3]);

    int rows = (int) Math.ceil(n / 2.0);
    int columns = (int) Math.ceil(m / 2.0);

    int maxPerColumn = m - r + 1;
    int maxPerRow = n - r + 1;
    HashSet<Pair> set = new HashSet<>();
    PriorityQueue<PQObject> pq = new PriorityQueue<>(new PQComparator());
    pq.add(new PQObject(getCellCount(rows, columns, n, m, r, maxPerColumn, maxPerRow), rows, columns));

    int count = k;
    long sum = 0;
    while(count > 0){
      PQObject pqObject = pq.poll();
      sum += pqObject.value;
      count--;
      int i = pqObject.row;
      int j = pqObject.column;
      boolean hasRowMirror = hasMirror(i, rows, n);
      boolean hasColumnMirror = hasMirror(j, columns, m);
      if(count > 0 && hasRowMirror){
        count--;
        sum += pqObject.value;
      }
      if(count > 0 && hasColumnMirror){
        count--;
        sum += pqObject.value;
      }
      if(count > 0 && hasColumnMirror && hasRowMirror){
        count--;
        sum += pqObject.value;
      }

      if(i - 1 >=0){
        Pair p = new Pair(i - 1, j);
        if(!set.contains(p)){
          set.add(p);
          pq.add(new PQObject(getCellCount(i - 1, j, n, m, r, maxPerColumn, maxPerRow), i - 1, j));
        }
      }
      if(j - 1 >= 0){
        Pair p = new Pair(i, j - 1);
        if(!set.contains(p)){
          set.add(p);
          pq.add(new PQObject(getCellCount(i, j - 1, n, m, r, maxPerColumn, maxPerRow), i, j - 1));
        }
      }
    }

    System.out.println(sum / (1.0 * (n - r + 1) * (m - r + 1)));
  }


  private static long getCellCount(int i, int j, int n, int m, int r, int maxPerColumn, int maxPerRow){
    int left = j - r + 1 < 1 ? 1 : j - r + 1;
    int right = j + r - 1 > m ? m : j + r - 1;
    int up = i - r + 1 < 1 ? 1 : i - r + 1;
    int down = i + r - 1 > n ? n : i + r - 1;
    return getCount(j, left, right, maxPerColumn) * getCount(i, up, down, maxPerRow);
  }

  private static long getCount(int index, int lower, int upper, int max){
    return Math.min(max, Math.min(index - lower + 1, upper - index + 1));
  }
  private static boolean hasMirror(int index, int max, int rc){
    return rc - index + 1 > max;
  }
}