#include <fstream>
#include <iomanip>
#include <ctime>
#include <cstdlib>
using namespace std;
ifstream in("cifre.in");
ofstream out("cifre.out");
 
const int nmax = 1000000;
int a, b, c, k, dif, cnt;
double rasp;
 
bool Check(int x) 
{
	int cnt = 0;
	do {
		cnt += (x%10 == c);
		x /= 10;
    } while (x);

  return cnt>=k;
}
 
int main() {
	int player_unu=0;

	in>>a>>b>>c>>k;

	dif = b - a + 1;
	if(dif<=nmax)
	{
		for(int i = a; i<=b; i++)
			cnt += Check(i);
		rasp = double(cnt) / dif;
    }
	else
	{
		srand(time(0));
		for(int i = 0; i<nmax; i++)
			cnt += Check(rand() % dif + a);
		rasp = double(cnt) / nmax;
	}

	out<<setprecision(4)<<fixed<<rasp<< "\n";

	return player_unu;
}