var a,b,k,c,d,i,l,m,z:longint;
    n:real;
    f,g:text;
begin
assign(f,'cifre.in');
reset(f);
assign(g,'cifre.out');
rewrite(g);
read(f,a);
read(f,b);
read(f,k);
read(f,c);
for i:=a to b do begin
                 d:=i;
                 l:=0;
                       repeat
                       if d mod 10=c then l:=l+1;
                       d:=d div 10;
                       until (d=0) or (l=k);
                 if l=k then m:=m+1;
                 end;
z:=b-a+1;
n:=m/z;
write(g,n:4:4);
close(f);
close(g);
end.