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.*;
import java.util.*;
import java.math.*;
import java.util.concurrent.*;

public final class fishes
{
    static BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
	static FastScanner sc=new FastScanner(br);
    static PrintWriter out=new PrintWriter(System.out);
	static Random rnd=new Random();
	static long l_first,r_first;
	
	static int get(int m,int idx)
	{
		if(m%2==0)
		{
			if(idx%2==0)
			{
				return ((m/2)+1)+(idx/2)-1;
			}
			
			else
			{
				return (m/2)-(idx/2);
			}
		}
		
		else
		{
			if(idx%2==0)
			{
				int centre=(m/2)+1;
				
				return centre-(idx/2);
			}
			
			else
			{
				int centre=(m/2)+1;
				
				return centre+(idx/2);
			}
		}
	}
	
	static long get_w(long i,long col_idx,long r)
	{
		long val1=Math.max(1,i-r+1),val2=Math.max(1,col_idx-r+1);
			
		long xx=Math.min(l_first,i),yy=Math.min(r_first,col_idx);
			
		long mul1=Math.max(0,xx-val1+1),mul2=Math.max(0,yy-val2+1);
			
		long w=mul1*mul2;
		
		return w;
	}
	
    public static void main(String args[]) throws Exception
    {
		int n=sc.nextInt(),m=sc.nextInt(),r=sc.nextInt(),k=sc.nextInt();
		
		PriorityQueue<Node> pq=new PriorityQueue<>();
		
		l_first=n-r+1;r_first=m-r+1;
		
		for(int i=1;i<=n;i++)
		{
			int col_idx=(m/2)+(m%2);
			
			long w=get_w(i,col_idx,r);
			
			pq.add(new Node(i,col_idx,1,w));
			
			//ut.println(i+" "+col_idx+" "+w);
		}
		
		double xx=(n-r+1),yy=(m-r+1);
		
		double num=0,den=xx*yy;
		
		while(k>0)
		{
			Node curr=pq.poll();
			
			//out.println(curr.row+" "+curr.col+" "+curr.w);
			
			num+=curr.w;
						
			if(curr.idx<m)
			{	
				int new_idx=get(m,curr.idx+1);
				
				pq.add(new Node(curr.row,new_idx,curr.idx+1,get_w(curr.row,new_idx,r)));
			}
			
			k--;
		}
		
		double res=num/den;
		
		out.printf("%.20f\n",res);out.close();
    }
	
	private static class Node implements Comparable<Node>
	{
		int row,col,idx;long w;
		
		public Node(int row,int col,int idx,long w)
		{
			this.row=row;this.col=col;this.idx=idx;this.w=w;
		}
		
		public int compareTo(Node x)
		{
			if(this.w==x.w)
			{
				if(this.idx==x.idx)
				{
					return Integer.compare(this.col,x.col);
				}
				
				return Integer.compare(this.idx,x.idx);
			}
			
			return -(Long.compare(this.w,x.w));
		}
	}
}
class FastScanner
{
    BufferedReader in;
    StringTokenizer st;

    public FastScanner(BufferedReader in) {
        this.in = in;
    }
	
    public String nextToken() throws Exception {
        while (st == null || !st.hasMoreTokens()) {
            st = new StringTokenizer(in.readLine());
        }
        return st.nextToken();
    }
	
	public String next() throws Exception {
		return nextToken().toString();
	}
	
    public int nextInt() throws Exception {
        return Integer.parseInt(nextToken());
    }

    public long nextLong() throws Exception {
        return Long.parseLong(nextToken());
    }

    public double nextDouble() throws Exception {
        return Double.parseDouble(nextToken());
    }
}