#include"fstream.h"

int main()
{
	ifstream f("cifre.in");
   ofstream o("cifre.out");
   unsigned long a, b;
   unsigned int c, k, i, n, t, cf=0;
   f>>a>>b>>c>>k;
   for(i=a; i<=b; i++)
   {
   	t=0;
   	n=i;
      do{
      	if(n%10==c)
         	t++;
         n/=10;
      }while(n);
      if(t==k)
      	cf++;
   } if(a<10) cf++;
   o<<(float)(cf)/(b-a+1);
   f.close();
   o.close();
	return 0;
}