def main():
    N = int(input())
    spaces = []

    max_space = 0
    for _ in range(N):
        a, b = map(int, input().split())
        space = a + 4 * b
        spaces.append(space)
        max_space = max(max_space, space)

    diffs = 0
    for space in spaces:
        diff = max_space - space
        if diff % 2 != 0:
            print(-1)
            return
        diffs += diff

    print(diffs // 2)


if __name__ == "__main__":
    main()