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.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.PriorityQueue;
import java.io.IOException;
import java.util.InputMismatchException;
import java.io.InputStreamReader;
import java.util.StringTokenizer;
import java.util.Queue;
import java.io.BufferedReader;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author Darkhan Imangaliyev
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        QuickScanner in = new QuickScanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        TaskD solver = new TaskD();
        solver.solve(1, in, out);
        out.close();
    }

    static class TaskD {
        int n;
        int m;
        int r;

        public void solve(int testNumber, QuickScanner in, PrintWriter out) {
            n = in.nextInt();
            m = in.nextInt();
            r = in.nextInt();
            int k = in.nextInt();

            int MIDDLE = (m + 1) / 2;
            Queue<DataPoint> pq = new PriorityQueue<>();
            for (int i = 1; i <= n; ++i) {
                DataPoint top = new DataPoint(i, MIDDLE + 1, 1);
                DataPoint bottom = new DataPoint(i, MIDDLE, 0);
                pq.add(top);
                pq.add(bottom);
            }

            long ans = 0;
            for (int i = 0; i < k; ++i) {
                DataPoint top = pq.poll();
                ans += top.cells;

                if (top.side == 0) {
                    if (top.y - 1 > 0) {
                        pq.add(new DataPoint(top.x, top.y - 1, 0));
                    }
                } else {
                    if (top.y + 1 <= m) {
                        pq.add(new DataPoint(top.x, top.y + 1, 1));
                    }
                }
            }

            out.println(1. * ans / (n - r + 1) / (m - r + 1));
        }

        private static long countCells(int x, int y, int r, int n, int m) {
            int lo = Math.max(1, x - r + 1);
            int hi = Math.min(x + r - 1, n);
            int w = hi - lo + 1 - r + 1;

            lo = Math.max(1, y - r + 1);
            hi = Math.min(y + r - 1, m);
            int h = hi - lo + 1 - r + 1;

            return (long) w * h;
        }

        class DataPoint implements Comparable<DataPoint> {
            int x;
            int y;
            int side;
            long cells;

            public DataPoint(int x, int y, int side) {
                this.x = x;
                this.y = y;
                this.side = side;
                this.cells = countCells(x, y, r, n, m);
            }


            public String toString() {
                return "[x = " + x + ", y = " + y + ", side = " + side + ", cells = " + cells + "]";
            }


            public int compareTo(DataPoint other) {
                return Long.signum(other.cells - this.cells);
            }

        }

    }

    static class QuickScanner {
        BufferedReader in;
        StringTokenizer token;
        String delim;

        public QuickScanner(InputStream inputStream) {
            this.in = new BufferedReader(new InputStreamReader(inputStream));
            this.delim = " \n\t";
            this.token = new StringTokenizer("", delim);
        }

        public QuickScanner(InputStream inputStream, String delim) {
            this.in = new BufferedReader(new InputStreamReader(inputStream));
            this.delim = delim;
            this.token = new StringTokenizer("", delim);
        }

        public boolean hasNext() {
            while (!token.hasMoreTokens()) {
                try {
                    String s = in.readLine();
                    if (s == null) return false;
                    token = new StringTokenizer(s, delim);
                } catch (IOException e) {
                    throw new InputMismatchException();
                }
            }
            return true;
        }

        public String next() {
            hasNext();
            return token.nextToken();
        }

        public int nextInt() {
            return Integer.parseInt(next());
        }

    }
}