var  a,b,i,nr:longint;ap,p:integer;c,k:byte;f,g:text;
begin
assign(f,'cifre.in');reset(f);
assign(g,'cifre.out');rewrite(g);
readln(f,a,b,c,k);  p:=0;
for i:=a to b do
  begin
      nr:=i;ap:=0;{numara cate cifre c se gasesc}
         while nr<>0 do
          begin
           if (nr mod 10)=c then inc(ap);
           nr:=nr div 10;
           end;
           if ap=k then inc(p);
  end;
       write(g,p/(b-a):4:4);
       close(f);close(g);
       end.