#include<fstream.h>
#include<math.h>
#include<values.h>
#include<iomanip.h>
ifstream f("cifre.in");
ofstream g("cifre.out");
long a,b,caz_tot,caz_fav=0,i; int c,k; float probl;
int cif(int a,int c)
{
	k=0;
	while (a)
	{
		if (a%10==c) k++;
		a/=10;
	}
	return k;
}
int main ()
{
	f>>a>>b>>c>>k;
	caz_tot=b-a+1;
	for (i=a; i<=b; i++)
		if (cif(i,c)>=k) caz_fav++;
	probl=caz_fav/caz_tot;
	g<<setprecision(4)<<probl;
        return 0;
}