/**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;
		}
	}
}