import sys

def gcd(a, b):
    while b:
        a, b = b, a%b
    return a

lower = int(sys.stdin.readline().strip())
upper = int(sys.stdin.readline().strip())

i = lower
j = upper


while i <= j:
    res = gcd(i, j)
    if res == 1:
        print j - i + 1
        break
    i += 1
    j -= 1

if i >= j:
    print "-1"