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

parities = {s % 2 for s in s_list}
if len(parities) != 1:
    print(-1)
else:
    max_s = max(s_list)
    total = 0
    for s in s_list:
        total += (max_s - s) // 2
    print(total)