# -*- coding: utf-8 -*-
"""
No.178 美しいWhitespace (1)
https://yukicoder.me/problems/no/178

"""
import sys
from sys import stdin
input = stdin.readline


def solve(codes):
    codes.sort(reverse=True)
    ans = 0
    ref = codes[0]
    for c in codes[1:]:
        if (ref - c) % 2 != 0:
            ans = -1
            break
        ans += (ref - c) // 2
    return ans


def main(args):
    N = int(input())
    codes = []
    for _ in range(N):
        a, b = map(int, input().split())
        codes.append(a + 4 * b)
    ans = solve(codes)
    print(ans)

if __name__ == '__main__':
    main(sys.argv[1:])