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:
/* Author: Ronak Agarwal */
import java.io.* ; import java.util.* ; import java.math.* ;
import static java.lang.Math.min ;
import static java.lang.Math.max ;
import static java.lang.Math.abs ;
/* Thread is created here to increase the stack size of the java code so that recursive dfs can be performed */
public class Codeshefcode{
	public static void main(String[] args) throws IOException{
		new Thread(null,new Runnable(){	public void run(){ exec_Code() ;} },"Solver",1l<<27).start() ;
	}
	static void exec_Code(){
		try{
			Solver Machine = new Solver() ;
			Machine.Solve() ; Machine.Finish() ;}
		catch(Exception e){ e.printStackTrace() ; print_and_exit() ;}
		catch(Error e){ e.printStackTrace() ; print_and_exit() ; }
	}
	static void print_and_exit(){ System.out.flush() ; System.exit(-1) ;}
}
class Reader{
	BufferedReader reader;
	StringTokenizer tkz;
	Reader(InputStream input) {
		reader = new BufferedReader(new InputStreamReader(input)) ;
		tkz = new StringTokenizer("") ;
	}
	String s() throws IOException{
		while (!tkz.hasMoreTokens()) tkz = new StringTokenizer(reader.readLine()) ;
		return tkz.nextToken() ;
	}
	int i() throws IOException{ return Integer.parseInt(s()) ;}
	long l() throws IOException{ return Long.parseLong(s()) ;}
	double d() throws IOException{	return Double.parseDouble(s()) ;}
}
/* All the useful functions,constants,renamings are here*/
class Template{
	/* Constants Section */
	final int INT_MIN = Integer.MIN_VALUE ; final int INT_MAX = Integer.MAX_VALUE ;
	final long LONG_MIN = Long.MIN_VALUE ; final long LONG_MAX = Long.MAX_VALUE ;
	static long MOD = 1000000007 ;
	static Reader ip = new Reader(System.in) ;	
	static PrintWriter op = new PrintWriter(System.out) ;
	/* Methods for writing */
	static void p(Object o){ op.print(o) ; }
	static void pln(Object o){ op.println(o) ;}
	static void Finish(){ op.flush();	op.close(); }
	/* Implementation of operations modulo MOD */
	static long inv(long a){ return powM(a,MOD-2) ; }
	static long m(long a,long b){ return (a*b)%MOD ; }
	static long d(long a,long b){ return (a*inv(b))%MOD ; }
	static long powM(long a,long b){
		if(b<0) return powM(inv(a),-b) ;
		long val=a ; long ans=1 ;
		while(b!=0){
			if((b&1)==1) ans = (ans*val)%MOD ; 
			val = (val*val)%MOD ; 
			b/=2 ;
		}
		return ans ;
	}
	/* Renaming of some generic utility classes */
	final static class mylist extends ArrayList<Integer>{}
	final static class myset extends TreeSet<pair>{}
	final static class mystack extends Stack<Integer>{}
	final static class mymap extends TreeMap<Long,Integer>{}
}
/* Implementation of the pair class, useful for every other problem */
class pair implements Comparable<pair>{
	long x ; int y ;
	pair(long _x,int _y){ x=_x ; y=_y ;} 
	public int compareTo(pair p){
		return (this.x<p.x ? -1 : (this.x>p.x ? 1 : (this.y<p.y ? -1 : (this.y>p.y ? 1 : 0)))) ;
	}
}
/* Main code starts here */
class Solver extends Template{
	long[] weight(int n,int r){
		long evt[] = new long[n+1] ;
		long res[] = new long[n] ;
		for(int i=0 ; i<=(n-r) ; i++){
			evt[i]++ ;
			evt[i+r]-- ;
		}
		int val=0 ;
		for(int i=0 ; i<n ; i++){
			val+=evt[i] ;
			res[i]=val ;
		}
		return res ;
	}
	public void Solve() throws IOException{
		int n,m,r,k ;
		n = ip.i() ;
		m = ip.i() ;
		r = ip.i() ;
		k = ip.i() ;
		long wn[] = weight(n,r) ;
		long wm[] = weight(m,r) ;
		Arrays.sort(wn) ;
		Arrays.sort(wm) ;
		int ptr[] = new int[n] ;
		for(int i=0 ; i<n ; i++) ptr[i] = (m-1) ;
		myset set = new myset() ;
		for(int i=0 ; i<n ; i++)
			set.add(new pair(wn[i]*wm[ptr[i]],i)) ;
		long res=0 ;
		for(int itr=0 ; itr<k ; itr++){
			pair p = set.pollLast() ;
			res+=p.x ;
			int idx = p.y ;
			if(ptr[idx]>0){
				ptr[idx]-- ;
				set.add(new pair(wn[idx]*wm[ptr[idx]],idx)) ;				
			}
		}
		long poss = (n-r+1l)*(m-r+1l) ;
		pln(res/(poss+0.0)) ;
	}
}