def factorize(n): fct = [] div = 2 while div ** 2 <= n: cnt = 0 while n % div == 0: n //= div cnt += 1 if cnt > 0: fct.append((div, cnt)) div += 1 if n > 1: fct.append((n, 1)) return fct def dfs(fct, depth, cur): div, cnt = fct[depth] if depth == len(fct) - 1: return [cur*div**i for i in range(cnt+1)] rseq = [] for i in range(cnt+1): rseq += dfs(fct, depth+1, cur*div**i) return rseq A, B = map(int, input().split()) fct = factorize(A+B) C_cand = list(sorted(dfs(fct, 0, 1))) for C in C_cand: if (B + C) % A == 0 and (C + A) % B == 0: print(C) break else: print(-1)