def main():
    n = int(input())
    l = []
    for i in range(n):
        a, b = map(int, input().split())
        l.append(a + 4 * b)
    m = max(l)
    ans = 0
    for x in l:
        if (m - x) % 2 == 0:
            ans += (m - x) // 2
        else:
            ans = -1
            break
    print(ans)
    
if __name__ == '__main__':
    main()