#include<fstream>
#include<iomanip>
using namespace std;
int main()
{ifstream f("cifre.in");
ofstream g("cifre.out");
int c,k1,k;
long a,b,i,x;
double nr=0;
f>>a>>b>>k>>c;
for (i=a;i<=b;i++){
	x=i;k1=0;
	while (x!=0){
		if (x%10==c)
			k1++;
		x/=10;}
	if (k1>=k)
		nr+=1;}
g<<fixed<<setprecision(4)<<nr/(b-a+1);
f.close();
g.close();
return 0;}