#include<stdio.h>
#include<string.h>
#include<fstream.h>
ifstream f("cifre.in");
ofstream g("cifre.out");
int a,b,c,z,x,k,i,nr,n,cf,p;
float m;
char h;
int main()
{f>>a>>b>>c>>k;
 nr=0;
 for(i=a;i<=b;i++)
     {x=i;
      p=0;
      while(x!=0)
	    {cf=x%10;
	     x=x/10;
	     if(cf==c)
		p++; }
      if(p>=k)
	 nr++;}
 n=b-a+1;
 m=(float)nr/n;
 g<<m;
 f.close();
 g.close();
 return 0;}