var a,b,c,k:longint;
    s:string;
    f,g:text;

 procedure citire;
  begin
   read(f,a,b,c,k);
   str(c,s);
  end;

 function nrc(x:longint):boolean;
  var y:string;
      i,nr:longint;
      ok:boolean;
  begin
   str(x,y);
   i:=0;
   nr:=0;
   ok:=false;
   while (i<length(y)) and (not ok) do
     begin
     i:=i+1;
     if y[i]=s then nr:=nr+1;
     if nr=k then ok:=true;
     end;
   if ok then nrc:=true
   else nrc:=false;
  end;

 procedure cifre;
  var nr,i:longint;
  begin
   nr:=0;
   for i:=a to b do
     if nrc(i) then nr:=nr+1;
   write(g,nr/(b-a+1):0:4);
  end;

begin
  assign(f,'cifre.in');
  assign(g,'cifre.out');
  reset(f);
  rewrite(g);
  citire;
  cifre;
  close(f);
  close(g);
end.