n = int(input())
s = [0] * n
for i in range(n):
    a, b = map(int, input().split())
    s[i] = a + 4 * b
after = max(s)
ans = 0
for i in range(n):
    if (after - s[i]) % 2:
        print(-1)
        exit()
    ans += (after - s[i]) // 2
print(ans)