import sys
input = lambda : sys.stdin.readline().rstrip()

sys.setrecursionlimit(2*10**5+10)
write = lambda x: sys.stdout.write(x+"\n")
debug = lambda x: sys.stderr.write(x+"\n")
writef = lambda x: print("{:.12f}".format(x))

### 約数列挙
def fs(n):
    s = set()
    for i in range(1,int(n**0.5)+2):
        if n%i==0:
            s.add(i)
            s.add(n//i)
    l = sorted(list(s))
    return l

a,b = list(map(int, input().split()))
l = fs(a+b)
for v in sorted(l):
    if (a+v)%b==(b+v)%a==0 and a!=v and b!=v:
        print(v)
        break
else:
    print(-1)