#include<stdio.h>
int main()
{long a,b,fav=0,pos,i,x;
int c,k,r;
float p;
FILE *f=fopen("cifre.in","r");
FILE *g=fopen("cifre.out","w");
fscanf(f,"%ld %ld %d %d",&a,&b,&c,&k);
for (i=a;i<=b;i++)
	{x=i; r=0;
	while (x!=0&&r<=k) {if (x%10==c) r++;
				 x=x/10;}
	if (r>=k) fav++;}
pos=b-a+1;
p=(float)fav/pos;
fprintf(g,"%.4f",p);
fclose(f);
fclose(g);
return 0;}