Paired Seismic Magnitude Peak

A seismograph technician reviews a single day's log of aftershock readings. Each reading is a nonzero integer: a positive value k means the ground rose by k units at that instant, while a negative value -k means it sank by k units. No magnitude is ever logged twice with the same sign, so every reading in the log is distinct.

A magnitude k (k > 0) is called a matched swing if the log contains both the uplift reading k and the subsidence reading -k. The technician wants the single greatest magnitude among all matched swings recorded that day.

Input format

• Line 1: a single integer n, the number of readings in the log.
• Line 2: n space-separated distinct nonzero integers, the readings themselves.

Output format

Print a single integer: the greatest magnitude k such that both k and -k appear in the log. If no magnitude is matched, print -1.

Constraints

• 1 <= n <= 1000
• -1000 <= reading <= 1000, and no reading is 0
• All n readings are distinct

6
-2 5 1 -3 3 -5

5

4
-4 2 3 -2

2