def find_c(a,b,c):
    if (a+b) % c  == 0 and (b+c) % a == 0 and (c+a) % b == 0:
        return True
    else:
        return False

A,B = map(int,input().split())
C = -1
for i in range(1,A*B+1):
    if i == A or i == B:
        continue
    else:
        if find_c(A,B,i) == True:
            C = i
            break
print(C)