n = int(input())
A = []

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

c = max(A)
ans = 0
for a in A:
    if (c - a) % 2 != 0:
        ans = -1
        break
    ans += (c - a) // 2

print(ans)