#include<iostream.h>
#include<fstream.h>
#include<math.h>
#include <iomanip>
int i,j,c,a,b,k,uc,nr1,p;
float nr,s;

using namespace std;

main()
{ 
ifstream f("cifre.in");
ofstream g("cifre.out");
f>>a>>b;
f>>k>>c;	

for(i=a;i<=b;i++)
	{nr1=0;
		p=i;

	 while(p!=0)
	 {
		uc=p%10;
	    p=p/10;
	    //cout<<uc<<" ";
        if(uc==c)
	       nr1++;
		 if(nr1==k)
		   nr++;
	 }	
	//cout<<nr1;
	      
	}
	//cout<<nr<<" ";	
	
s=b-a+1;

	g<<fixed<<setprecision(4)<< nr/s;
}