#include<iostream>
#include<fstream>
#include <iomanip>
using namespace std;
int cifreIterativ(int numar, int cifra)
{
	int rezultat = 0;
	while (numar > 0)
	{
		int ultimaCifra = numar % 10;
		if (ultimaCifra == cifra)
		{
			rezultat++;
		}
		numar = numar / 10;
	}
	return rezultat;
};
int main()
{
	ifstream fin("cifre.in");
	ofstream fout("cifre.out");
	int A, B, C, K;
	float nr_aparitii = 0,nrCif=0;
	fin >> A >> B >> C >> K;
	for (int i = A; i <= B; i++)
	{
		nrCif=cifreIterativ(i, C);
		if(nrCif>=K)
		{nr_aparitii++; }
	}
	cout << fixed << setprecision(4) << nr_aparitii / B;
	return 0;
}