/**In my country, streets dont have names, each of them are

* just given a number as name. These numbers are supposed

* to be unique but that is not always the case. The local

* government allocates some integers to name the roads and

* in many case the number of integers allocated is less that

* the total number of roads. In that case to make road

* names unique some single character suffixes are used. So

* roads are named as 1, 2, 3, 1A, 2B, 3C, etc. Of course the

* number of suffixes is also always limited to 26 (A, B, . . . ,

* Z). For example if there are 4 roads and 2 different integers

* are allocated for naming then some possible assignments

* of names can be:

* 1, 2, 1A, 2B

* 1, 2, 1A, 2C

* 3, 4, 3A, 4A

* 1, 2, 1B, 1C

* Given the number of roads (R) and the numbers of

* integers allocated for naming (N), your job is to determine

* minimum how many different suffixes will be required (of

* all possible namings) to name the streets assuming that

* no two streets can have same names.

* Input

* The input file can contain up to 10002 lines of inputs. Each line contains two integers R and N

* (0 < N, R < 10001). Here R is the total number of streets to be named and N denotes number integers

* allocated for naming.

* Output

* For each line of input produce one line of output. This line contains the serial of output followed by

* an integer D which denotes the minimum number of suffixes required to name the streets. If it is not

* possible to name all the streets print ‘impossible’ instead (without the quotes).

* Sample Input

* 8 5

* 100 2

* 0 0

* Sample Output

* Case 1: 1

* Case 2: impossible

*/

//https://uva.onlinejudge.org/index.php?option=com_onlinejudge&Itemid=8&page=show_problem&problem=2823

import java.util.Scanner;

public class NumberingRoads {

public static void main(String[] args) {

Scanner input = new Scanner(System.in);

int caseNumber = 1;

while (true) {

int first = input.nextInt();

int second = input.nextInt();

if (first == 0 && second == 0) {

break;

}

boolean found = false;

for (int i = 0; i < 27 && !found; i++) {

int sum = second + second * i;

if (sum >= first) {

System.out.print("Case " + caseNumber + ": " + i + "\n");

found = true;

}

}

if (!found) {

System.out.print("Case " + caseNumber + ": impossible\n");

}

caseNumber = caseNumber + 1;

}

}

}