var a,b,c,k,cf:int64;
    i:longint;

function nr(n,x,y:integer):boolean;
    var aux,c:integer;
    begin
       while n<>0 do begin
       aux:=n mod 10;
       n:=n div 10;
       if x=aux then inc(c);
       if c>=y then nr:=TRUE else nr:=FALSE;

    end;
    c:=0;
   end;


begin
assign(input,'cifre.in'); reset(input);
assign(output,'cifre.out'); rewrite(output);
readln(a,b,c,k);
for i:=a to b do if nr(i,c,k) then inc(cf);
write(cf/(b-a+1):6:4);
close(input); close(output);
end.