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Points:
5 (partial)

Time limit:
2.0s

Java
5.0s

Python
5.0s

Memory limit:
64M

Java
64M

Python
64M

Author:

Problem type

Roger has a list of positive integers . However, his list is not yet finalized. Some of these numbers are *wildcards* which will be represented as zeroes in the list. Roger will try to assign the wildcards a value so that

- The list is sorted from least to greatest
- All wildcards have the same value, which is a positive integer

Help Roger find out if this is possible. Output `YES`

if he can assign the wildcards a value so that is sorted and `NO`

otherwise.

#### Constraints

for all

If , then it is a wildcard, otherwise .

#### Input Specification

The first line contains a single integer .

The next line contains space-separated integers, .

#### Output Specification

Output a single string: `YES`

if it is possible and `NO`

otherwise.

##### Subtask 1 [30%]

##### Subtask 2 [30%]

##### Subtask 3 [40%]

#### Sample Input 1

```
6
0 0 1 5 5 5
```

#### Sample Output 1

`YES`

#### Explanation for Sample Input 1

The first two elements are wildcards. Setting them to gives a sorted list.

#### Sample Input 2

```
6
1 5 5 5 5 1
```

#### Sample Output 2

`NO`

#### Explanation for Sample 2

There are no wildcards, so Roger cannot do anything. is not sorted so the answer is no.

#### Sample Input 3

```
6
1 0 2 0 3 0
```

#### Sample Output 3

`NO`

## Comments