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