n = int(input())
s_list = []
parity = None
possible = True

for _ in range(n):
    a, b = map(int, input().split())
    s = a + 4 * b
    s_list.append(s)
    if parity is None:
        parity = s % 2
    else:
        if s % 2 != parity:
            possible = False

if not possible:
    print(-1)
else:
    max_s = max(s_list)
    total = 0
    for s in s_list:
        diff = max_s - s
        total += diff // 2
    print(total)