#include <fstream.h>

int main ()
{
  ifstream f("cifre.in");
  ofstream g("cifre.out");
  long a,b,x,k,c,p,cc,nrc,nrtot;
  double prob;
  f>>a>>b>>c>>k;
  nrtot=0;
  x=a;
  while (x<=b)
    {
      p=x;
      nrc=0;
      do
	{
	  cc=p%10;
	  p=p/10;
	  if (cc==c) nrc=nrc+1;
	}
      while (p!=0);
      if (nrc>=k) nrtot=nrtot+1;
      x++;
    }
  g.precision(4);
  prob=(double)nrtot/(b-a+1);
  g<<prob<<"\n";
  f.close();
  g.close();
  return 0;
}