# -*- coding:utf-8 -*-
# http://yukicoder.me/problems/424
if __name__ == '__main__':
    n = int(input())
    sn = []
    sn.extend((lambda a, b: a + 4 * b)(*map(int, input().split())) for i in range(n))
    m = max(sn)
    cnt = 0
    for s in sn:
        d = m - s
        if d % 2 != 0:
            cnt = -1
            break
        cnt += d / 2
    print(int(cnt))