program cifre;
var f,g:text;
    a,b,nr,i,j,d:longint;
    s:string;
    sr:string[2];
    c,k,e:byte;
    p:real;
begin
assign(f,'cifre.in');assign(g,'cifre.out');reset(f);rewrite(g);
read(f,a,b,c,k);d:=0;str(c,sr);
for i:=a to b do begin
str(i,s); nr:=0;
for j:=1 to length(s) do
if s[j]=sr then begin nr:=nr+1;
if nr=k then begin d:=d+1;break;end;end;
end;
p:=d/(b-a+1);
write(g,p:0:4);close(f);close(g);end.