var a,b,t,i,nr2,nr,q:longint;
f,g:text;
k,c:0..9;

begin
assign(f,'cifre.in');reset(f);
assign(g,'cifre.out');reset(g);
read(f,a,b,c,k);
t:=a-b+1;
for i:=a to b do begin
nr:=0;
q:=i;
while q<>0 do begin
  if q mod 10=c then begin
    inc(nr);
    q:=q div 10;
  end
  else
  q:=q div 10;
end;
if nr>=k then
  inc(nr2);
end;
writeln(g,nr2/t:0:4);
close(f);
close(g);
end.