#include<stdio.h>
long A, B, i, ci,contor=0;
int C, K, nr=0;
float p;
FILE *f, *g;
int main()
{
	f=fopen("cifre.in","r");
	fscanf (f,"%ld%ld%d%d",&A, &B, &C, &K);
	fclose (f);
	for (i=A; i<=B; i++){ci=i;nr=0;
		while ((ci>0)&&(nr<K)) {if (ci%10==C) nr++; 
								ci=ci/10;}
		if (nr>=K) contor++;
	}
p=(float)contor/(B-A+1);
g=fopen("cifre.out","w");
fprintf (g,"%.4f",p);
fclose (g);
return 0;
}