Output Format

For each dataset, print one line $\text{\texttt{Case Number }\textit{N}\texttt{:}}$ (where $N$ starts from $1$) followed by an integer denoting the minimal total waiting time, or the word $\verb!impossible!$ if Maria cannot accomplish the task.

See the sample output.

4
55
5 10 15
4
0 5 10 20
4
0 5 10 15
4
18
1 2 3
5
0 3 6 10 12
6
0 3 5 7 12 15
2
30
20
1
20
7
1 3 5 7 11 13 17
0

Case Number 1: 5
Case Number 2: 0
Case Number 3: impossible

Hint

Explanation for Sample 1:

She boards at minute $0$, gets off at station $3$, immediately takes the train that left at minute $0$ and departs at minute $15$ to go back, gets to station $2$, immediately takes the train that started at minute $20$ and departs at minute $25$ to the terminal, arrives at minute $50$, and then needs to wait $5$ minutes.

Translated by ChatGPT 5