n = int(input())

l = []
for _ in range(n):
    a, b = map(int, input().split())
    l.append(a + 4*b)

y = max(l)
z = 0
for i in l:
    if (y-i) % 2 == 0:
        z += (y-i) // 2
    else:
        z = -1
        break

print(z)