#include <iostream>
#include <fstream>
using namespace std;

ifstream f("cifre.in");
ofstream g("cifre.out");

int a,b,c,k;

int verif(int q)
{
	int nr = 0;
	while(q)
	{
		if(q%10 == c)
			nr++;
		if(nr>=k)
			return 1;
		q /= 10;
	}
	return 0;
}

int main()
{
	f>>a>>b>>c>>k;
	int x = 0;
	for(int i = a; i<= b; i++)
		if(verif(i)) x++;
	int aux = b-a+1;
	g<<(double)x/(double)aux;
	return 0;
}