n = int(input())
width = [0] * n
even = False
odd = False
for i in range(n):
    a, b = [int(x) for x in input().split()]
    width[i] = a + 4 * b
    if a % 2 == 0:
        even = True
    else:
        odd = True

res = 0
if even and odd:
    res = -1
else:
    len_max = max(width)
    for w in width:
        res += len_max - w
    res //= 2

print(res)