import math
import sys


def input(): return sys.stdin.readline().rstrip('\n')


def main():
    n = int(input())
    c = []
    for i in range(n):
        a, b = map(int, input().split())
        c.append(a+4*b)
    mx = max(c)

    z = 0
    for a in c:
        a = mx-a
        if a % 2 == 1:
            print(-1)
            return
        z += a // 2

    print(z)


if __name__ == '__main__':
    main()