# coding:utf-8

N = int(input())
s1 = []
s2 = []

for i in range(N):
    a, b = map(int, input().split())
    c = a + 4 * b
    s1.append(c)
    s2.append(c%2)
if len(set(s2)) == 2:
    print('-1')
else:
    count_fullwidth = 0
    for j in s1:
        count_fullwidth += (max(s1) - j) // 2
    print(count_fullwidth)