#include<fstream>
using namespace std;
int main()  
{  
	long A,B,i,x;
	int c,k,nr;
	float p,j=0;
	ifstream in("cifre.in");  
    ofstream out("cifre.out"); 	
	in>>A; in>>B; in>>c; in>>k;
	for (i=A;i<=B;i++)
	{
		nr=0;
		x=i;
		while (x!=0)
		{
			if (x%10==c) nr+=1;
			x/=10;
		}
		if (k<=nr) j+=1;
	}
	p=j/(B-A+1);
	out<<p;
	in.close();  
    out.close();  
    return 0;
}