Problem 5470. -- [Usaco2017 Open]Modern Art 2

## 5470: [Usaco2017 Open]Modern Art 2

Time Limit: 1 Sec  Memory Limit: 128 MB
Submit: 12  Solved: 7
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## Description

Having become bored with standard 2-dimensional artwork (and also frustrated at others copying her w
ork), the great bovine artist Picowso has decided to switch to a more minimalist, 1-dimensional styl
e.Although, her paintings can now be described by a 1-dimensional array of colors of length N(1≤N≤
100,000), her painting style remains unchanged: she starts with a blank canvas and layers upon it a
sequence of "rectangles" of paint, which in this 1-dimensional case are simply intervals. Sheuses ea
ch of the colors 1…Nexactly once, although just as before, some colors might end up being completel
y covered up by the end.To Picowso's great dismay, her competitor Moonet seems to have figured out h
ow to copy even these 1-dimensional paintings, using a similar strategy to the preceding problem: Mo
onet will paint a set ofdisjoint intervals, wait for them to dry, then paint another set of disjoint
intervals, and so on.Moonet can only paint at most one interval of each color over the entire proce
ss. Please compute thenumber of such rounds needed for Moonet to copy a given 1-dimensional Picowso
painting.

## Input

The first line of input contains N, and the next N lines contain an integer in the range 0…N
indicating the color of each cell in the 1-dimensional painting (0 for a blank cell).

## Output

Please output the minimum number of rounds needed to copy this painting, or -1 if this could not hav
e possibly been an authentic work of Picowso (i.e., if she could not have painted it using a layered
sequence of intervals, one of each color).

## Sample Input

7

0

1

4

5

1

3

3

## Sample Output

2

## HINT

In this example, the interval of color 1 must be painted in an earlier round than the intervals of c

olors 4 and 5, so at least two rounds are needed.

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