#include <stdio.h>

long a,b,i,m,p;
int c,k,ap[9],uc;
float n;

int main ()
{
  freopen("cifre.in","r",stdin);
  freopen("cifre.out","w",stdout);
  scanf("%ld" "%ld\n", &a, &b);
  scanf("%d" "%d", &c, &k);
  for (i=a;i<=b;i++)
	{
	  p=0;
	  m=i;
	  while (m!=0)
		{
		  uc=m%10;
		  if(uc==c) p++;
		  m/=10;
		 }
	  if (p>=k) ap[c]++;
	 }
  n=(float)ap[c]/(i-1);
  printf("%.4f", n);
  return 0;
}