# a + b = z * c
# b + c = x * a
# c + a = y * b
# 両辺をそれぞれ足し合わせると
# 2 * a + 2 * b + 2 * c = x * a + y * b + z * c
# <=> (2 - x) * a + (2 - y) * b + (2 - z) * c = 0
# a, b, cは自然数より、x = 2, y = 2, c = 2
# よって a * b = 2 * c 

a, b = map(int, input().split())
if (a + b) % 2 == 0:
    print((a + b) // 2)
else:
    print(-1)