//"Nu specific toate detaliile" - Explicatie oficiala

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

ifstream fin ("cifre.in");
ofstream fout ("cifre.out");

const int R = 950000;

int a, b, c, k, sol;

int get(int x) {
	int now = 0;
	while (x) {
		if (x % 10 == c)
			now++;
		x /= 10;
	}
	if (now >= k)
		return 1;
	return 0;
}

int main() {
	fin >> a >> b >> c >> k;
	if (b - a <= 1e6 + 1)
		for (int x = a; x <= b; ++x)
			sol += get(x);
	if (sol) {
		fout << 1.0 * sol / (b - a + 1);
		return 0;
	}
	srand ((unsigned) time(0));
	for (int r = 0; r < R; ++r) {
		int x = a + rand() % (b - a + 1);
		sol += get(x);
	}
	fout << 1.0 * sol / R ;			
}