def main():
    N = int(input())
    code_lines = [list(map(int, input().split())) for _ in range(N)]

    code_letter_num = [a + 4*b for (a, b) in code_lines]
    max_code_line = max(code_letter_num)
    if any((max_code_line - letter_num) % 2 != 0 for letter_num in code_letter_num):
        print(-1)
    else:
        print(sum((max_code_line - letter_num)
              for letter_num in code_letter_num) // 2)


if __name__ == "__main__":
    main()