var f,g:text;
        a,b,i,n,m:real;
        c,k,j:byte;
begin

        assign(f,'cifre.in');
        reset(f);
        read(f,a);
        read(f,b);
        read(f,c);
        read(f,k);
        close(f);

        m:=0;
        for i:=a to b do begin
               n:=i;
               j:=0;
               repeat
                        if n mod 10=c then inc(j);
                        n:=n div 10;
               until n=0;
               if j>=k then inc(m);
        end;

        assign(g,'cifre.out');
        rewrite(g);
        writeln(g,m/b-a:1:4);
        close(g);
end.