#include <fstream>
#include <iomanip>
using namespace std;
ifstream f("cifre.in");
ofstream g("cifre.out");
long a,b,i,aux,nrc,nr,k,c;
double r;
int main()
{
	f>>a>>b>>c>>k;
	for (i=a;i<=b;i++)
	{
		aux=i;
		nrc=0;
		while (aux!=0)
		{
			if (aux%10==c)
				nrc++;
			aux=aux/10;
		}
		if (nrc>=k)
			nr++;
	}
	r=1.0*nr/(b-a+1);
	g<<setprecision(4);
	g<<r;
	f.close();
	g.close();
	return 0;
}