#include <stdio.h>
FILE *f=fopen("cifre.in", "r"), *g=fopen("cifre.out", "w");
long x, i, a, b, c, k, sw, total, t;


void citeste(void)
	{
   fscanf(f, "%ld%ld%ld%ld", &a, &b, &c, &k);
	total=b-a+1;
   }


void rezolva(void)
	{
   for (i=a;i<=b;i++)
   	 {
       sw=0;
       t=i;
       while (t)
       	{
         if (t%10==c)
         	sw++;
         if (sw==k)
         	{
            x++;
            break;
            }
         t*=0.1;
         }
       }
   }



void tipareste(void)
	{
   fprintf(g, "%.4f", (float)x/total);
   fcloseall();
   }   

int main(void)
{
citeste();
rezolva();
tipareste();
return 0;
}