#include <fstream.h>
#include <iomanip.h>


unsigned long int a, b;
short int c, k;
int Ver(int, int, int);



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


	fin >> a >> b >> c >> k;

	int fav = 0, pos = 0;

	for (int i = a; i <= b; i++)
	{
		if (Ver(i, c, k))
		{
			fav++;
			pos++;
		}
		else
			pos++;
	}


	fout << setprecision(4) << (float)fav / pos;


	fin.close();
	fout.close();
	return 0;
}

int Ver(int x, int y, int z)
{
	int nr = 0;

	while(x)
	{
		if (x %10 == y)
			nr++;
		x /= 10;
	}
	if (nr >= z)
		return 1;
	else return 0;
}