import sequtils,strutils,rationals
let
  N = stdin.readLine().parseInt()
  M = stdin.readLine().parseInt()
  r = (N // M) * (1//1)
  (n,m) = (r.num,r.den)
if m == 1: echo(($n).filterIt(it != '0')[^1]) # 200 => 2に
elif m == 2 : echo 5
elif m == 5 : echo(2 * (n mod 5))
else: echo -1