# -*- coding: utf-8 -*-
N = input()
SPN = [0] * N
cnt = 0
for i in range(0,N):
    A, B = map(int, raw_input().split())
    SPN[i] = A + 4 * B
maxSPN = max(SPN)
for i in range(0,N):
    if (maxSPN - SPN[i]) % 2 == 1:
        print -1
        exit()
    else:
        cnt += (maxSPN - SPN[i]) // 2
print cnt