import sys

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


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

i = lower
j = upper

found = False

for i in range(lower, upper):
    for j in range(upper, lower, -1):
        if i % 2 == 1 or j % 2 == 1:
            res = gcd(i, j)
            if res == 1:
                print j - i + 1
                found = True
                break
    if found:
        break

if i >= j:
    print -1