#include<fstream.h>
unsigned  a,i,m,d=0,j,b,p,c,k,ok=0;
float l;
int main()
{ifstream f("cifre.in");
ofstream g("cifre.out");
f>>a>>b>>c>>k;l=0;i=a;
while(i<=b)
 {m=0;p=i;
  while((p!=0)||(ok==0))
   {if((p%10)==c)
      m++;
    p=p/10;
    if(m==k)
     ok=1;
   }

   if(m>=k)
     d++;
 i++;
 }
l=b-a+1;
g<<float(d/l);
f.close();g.close();
return 0;
}