var a,b,c,i,j,l,w,k,t:longint;
    f,g:text;
begin
 assign(f,'cifre.in');
 reset(f);
 read(f,a);
 read(f,b);
 read(f,c);
 read(f,k);
 close(f);
 w:=0;
 l:=0;
 if k<3 then
 for i:=a to b do begin
  inc(l);
  j:=i;
  t:=0;
  repeat
   if j mod 10=c then inc(t);
   j:=j div 10;
   until j<=0;
  if t>=k then inc(w);
  end
  else w:=1;
 assign(g,'cifre.out');
 rewrite(g);
 write(g,w/l:10:4);
close(g);
end.