27622번 - Suspicious Event 다국어

Before you graduate from your beloved university, you managed to secure an internship at a security company. Your first task is to analyze a login/logout sequence.

You are given a sequence of integers A that represents a record for users login/logout events in chronological order. Each user is represented by a unique integer. A positive integer x indicates a login event while a negative integer −x indicates a logout event by user |x|. You are guaranteed that there are no two events that are the same in A.

Once in a while, you might find from the record, a user who logs out even though that user has never been logged in previously. This can be due to a bug in the system or a hacking attempt. Nevertheless, it’s suspicious!

For example, let A_1..8 = (8, 3, −8, −6, 7, −3, −1, 6). There are 8 events from 5 distinct users (the users are 1, 3, 6, 7, and 8). The events are as follows.

• A₁ = 8 → user 8 log in.
• A₂ = 3 → user 3 log in.
• A₃ = −8 → user 8 log out.
• A₄ = −6 → user 6 log out – suspicious event! (user 6 has not logged in)
• A₅ = 7 → user 7 log in.
• A₆ = −3 → user 3 log out.
• A₇ = −1 → user 1 log out – suspicious event! (user 1 has not logged in)
• A₈ = 6 → user 6 log in.

In total, there are 2 suspicious logout events, i.e. A₄ = −6 and A₇ = −1. Note that although user 6 logs in at A₈, he has not logged in when he logs out at A₄.

Your task in this problem is to count how many logout suspicious events are there given the login/logout record.

Input begins with a line containing an integer N (1 ≤ N ≤ 1000) representing the number of events in the given record. The next line contains N integers A_i (−1000 ≤ A_i ≤ 1000; A_i ≠ 0) each representing an event in chronological order. It is guaranteed that all integers in A are distinct.

Output contains an integer in a line representing the number of suspicious logout events.