n = int(input())

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

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