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 A != C and B != C and (B + C) % A == 0 and (C + A) % B == 0:
        print(C)
        break
else:
    print(-1)