var a,b,i,p:longint;
    c,k,nr,nr2:byte;
    f,g:text;
begin
assign(f,'cifre.in');reset(f);
read(f,a,b,c,k);
close(f);
for i:=a to b do begin
   nr:=0;p:=i;
   while p<>0 do begin
         if p mod 10=c then nr:=nr+1;
         p:=p div 10;
         end;
   if k<=nr then inc(nr2);
   end;
assign(g,'cifre.out');rewrite(g);
writeln(g,nr2/(b-a+1):1:4);
close(g);
end.