n = int(input())

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

max = max(list)

counter = 0
for e in sorted(list):
    if (max - e) % 2 == 0:
        counter += max - e
    else:
        print(-1)
        break
else:
    print(int(counter / 2))