#!/usr/bin/env python
#coding:utf8


def read():
    N = input()
    ret = []
    for i in range(N):
        ret.append(map(int, raw_input().split()))
    return ret


def work(ABList):
    vList = [A + B * 4 for (A, B) in ABList]

    maxV = max(vList)

    ans = 0
    for (A, B) in ABList:
        if (maxV - (A + B * 4)) % 2 != 0:
            print -1
            return
        ans += (maxV - (A + B * 4)) / 2

    print ans


if __name__ == "__main__":
    work(read())