Suspicious Event

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 records the login and logout events of users in chronological order. Each user is represented by a unique integer. A positive integer $x$ is a login event by user $x$, and a negative integer $-x$ is a logout event by user $|x|$. No two events in $A$ are the same.

Sometimes the record shows a user who logs out without having logged in before. This can be caused by a bug in the system or by a hacking attempt. Either way, such an event is suspicious.

For example, let $A_{1..8} = (8, 3, -8, -6, 7, -3, -1, 6)$. There are 8 events from 5 distinct users (users 1, 3, 6, 7, and 8).

• $A_1 = 8$: user 8 logs in.
• $A_2 = 3$: user 3 logs in.
• $A_3 = -8$: user 8 logs out.
• $A_4 = -6$: user 6 logs out. This is a suspicious event, because user 6 has not logged in.
• $A_5 = 7$: user 7 logs in.
• $A_6 = -3$: user 3 logs out.
• $A_7 = -1$: user 1 logs out. This is a suspicious event, because user 1 has not logged in.
• $A_8 = 6$: user 6 logs in.

There are 2 suspicious logout events, $A_4 = -6$ and $A_7 = -1$. User 6 logs in at $A_8$, but that is after the logout at $A_4$, so $A_4$ is still suspicious.

Your task is to count the suspicious logout events in the record.

The first line contains an integer $N$ ($1 \le N \le 1000$), the number of events. The second line contains $N$ integers $A_i$ ($-1000 \le A_i \le 1000$, $A_i \ne 0$), the events in chronological order. All integers in $A$ are distinct.

Print one integer on a line: the number of suspicious logout events.