N = int(input())
ans = 0
list = []

for i in range(N):
    a, b = map(int, input().split())
    sum_b = b * 4
    sum = a + sum_b
    list.append(sum)

for g in list:
    if (max(list) - g) % 2 == 0:
        ans += (max(list) - g) / 2

    else:
        ans = -1
        break

print(int(ans))