//"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 = 850000;

int a, b, c, k, sol;

int main() {
	fin >> a >> b >> c >> k;
	srand ((unsigned) time(0));
	for (int r = 0; r < R; ++r) {
		int x = a + rand() % (b - a + 1), now = 0;
		while (x) {
			if (x % 10 == c)
				now++;
			x /= 10;
		}
		if (now >= k)
			sol++;
	}
	fout << 1.0 * sol / R ;			
}