import strutils, tables

proc input: string =
  while true:
    let t = stdin.readChar
    if t.isSpaceAscii: break else: result.safeAdd t

let n, m = input().parseInt
var dic = initCountTable[string]()

for i in 0 ..< m:
  dic.inc (i * 2 mod n).repr

if (dic.len * 2) mod n != 0:
  echo -1
else:
  for m in countdown(m, 2):
    if n mod m == 0:
      echo n div m
      quit()