# coding:utf-8

N = int(input())
ss = []
s2 = []


for i in range(N):
    a, b = map(int, input().split())
    c = a + 4 * b
    ss.append(c)
    s2.append(c%2)
if len(set(s2)) == 2:
    print('-1')
else:
    result = 0
    sum_space = max(ss)
    for j in ss:
        result += (sum_space - j) // 2
    print(result)