import sys

a = int(raw_input())
b = int(raw_input())

def hcf( n1, n2 ):
  while n1*n2:
    if n1 > n2:
      n1 %= n2
    else:
      n2 %= n1
  return max( n1, n2 )

def coprime( n1, n2 ):
  return hcf( n1, n2 ) == 1

while b > a:
    x1 = b
    x2 = a

    if coprime(x1, x2):
        print x1 - x2 + 1
        sys.exit(0)
    b -= 1
    a += 1
else:
    print -1
    sys.exit(0)