#!/usr/bin/env python
# -*- coding: utf-8 -*-

f = lambda x:x[0] + 4*x[1]
N = int(input())
lines = [f(list(map(int,input().split()))) for i in range(N)]
max_size = max(lines)
count = 0

for l in lines:
    if (max_size-l)%2 != 0:
        print(-1)
        quit()
    else:
        count += (max_size-l)//2
else:
    print(count)