#!/usr/bin/env python3
#fileencoding: utf-8

max_spaces = 0
spaces = []
for i in range(int(input())):
    a, b = [int(i) for i in input().strip().split(" ")]
    tmp = a + b * 4
    if max_spaces < tmp:
        max_spaces = tmp
    spaces.append(tmp)

count = 0
for i in spaces:
    if ((max_spaces - i) % 2) != 0:
        count = -1
        break
    else:
        count += (max_spaces - i) // 2

print(count)