#include<iostream>
#include<fstream>
using namespace std;
ifstream f("cifre.in");
ofstream g("cifre.out");
int cif (int x, long a)
{int r=0;
	while (a!=0)
	{
		if (a%10==x)
			r++;
		a=a/10;
	}
	return r;
}
int main()
{
	long double A,B,i;
	float K, C,j=0;
	f>>A>>B>>C>>K;
	for (i=A; i<=B;i++)
		if (cif(C,i)>=K)
			j++;
	g<<j/(B-A+1);
	return 0;
}