def main():
    import sys
    from fractions import gcd
    input = sys.stdin.readline

    a, b = map(int, input().split())

    if gcd(a, b) != 1:
        print(-1)
        exit()

    ans = [1] * (a*b+1)
    for x in range(b+1):
        for y in range(a+1):
            n = a*x+b*y
            if n <= a*b:
                ans[n] = 0
    print(sum(ans))


if __name__ == '__main__':
    main()