def main():
    candidates = None
    for _ in range(int(input())):
        X, Y = map(int, input().split())
        if X >= Y:
            print(-1)
            return

        if candidates is None:
            candidates = Y - X
            continue

        if candidates != Y - X:
            print(-1)
            return

    if candidates is not None:
        print(candidates)
    else:
        print(-1)


if __name__ == "__main__":
    main()