#include<fstream>
using namespace std;
int a,b,c,k,i,nr,x;
float s,ap=0,m;
int main ()
{
ifstream in ("date.in");
ofstream out("date.out");
in>>a;
in>>b;
in>>c;
in>>k;
for (i=a;i<=b;i++)
{ m++;
  nr=i;
  while (nr!=0)
  { x=nr%10;
    if (x==c) {ap++;break;}
    else nr=nr/10; } }
if (ap>=k)
{ s=ap/m;
 out<<"probabilitatea e de "<<s; }
 in.close();
 out.close();
return 0;
}