#include<fstream.h>
#include<iostream.h>
using namespace std;
unsigned long long t;
short int y;
int cif (unsigned long long t, short int y)
{
	int cifre, a;
	cifre=0;
	while (t!=0)
	{
		a=t%10;
		if (a==y)
			cifre=cifre+1;
		t=t/10;
	
	}
return cifre;
}
int main()
{
	unsigned long long a, b, i, x, nr, d;
	short int k, c, cifre;
	float raport;
	ifstream f("cifre.in");
	ofstream h("cifre.out");
	f>>a;
	f>>b;
	f>>c;
	f>>k;
	d=b-a+1;
	nr=0;
	for (i=a; i<=b; i++)
	{
		x=i;
		cifre=cif(i, c);
		if (cifre>=k)
			nr=nr+1;
	
	}
	raport=float(nr)/float(d);
	h.precision(4);
	h<<raport;
	h<<'\n';
	f.close();
	h.close();
	return 0;


}