Popularity in the Facebook
By Marcio T. I. Oshiro 
Brazil
Timelimit: 3
Brazil BrazilNowadays everyone is connected, take part in the Facebook, publish your photos on Instagram, your videos on YouTube, and so on. Even as GPS systems today are based on social networks, making it fun (and perhaps more difficult to understand, but that's another conversation). Being popular on Facebook is almost a necessity. A person with less than 700, 800 friends can be considered almost as a pariah in this new reality.
Maybe that's why some people tend to exaggerate when they say the number of friends they have. Consider a community of N individuals, and for each one, consider that we know the number of friends that everyone claims to have in the community. Your task in this problem is to determine if indeed it is possible that all community members are speaking the truth. Remember that a person can not be a friend to herself, and the two people can't be friends several times.
The input contains many instances and ends with EOF. The first line of each instance contains an integer N (1 ≤ N ≤ 105). The second line contains N integers, ai (0 ≤ ai ≤ 105), separated by an blank space, corresponding to the number ofr friends that the i people said have in the community.
For each instance, print in a only line, possivel if it is possible that all the people of that community are saying the truth, or otherwise, impossivel.
| Sample Input | Sample Output |
|
3 |
impossivel |