var a,b,i,n,ct,ct1:longint;
    k,uc,c:byte;
    t:real;
    f1,f2:text;
begin

        assign(f1,'cifre.in');
        reset(f1);
        assign(f2,'cifre.out');
        rewrite(f2);

        read(f1,a,b,c,k);

        ct1:=0;
        for i:=a to b do
        begin
                ct:=0;
                n:=i;

                while n<>0 do
                begin

                uc:=n mod 10;
                n:=n div 10;

                if uc=c then inc(ct);

                if ct=k then begin
                                inc(ct1);break;end;

                end;
        end;
        t:=ct1/(b-a+1);
        write(f2,t:8:4);
        close(f1);
        close(f2);
end.