/**

          题意：给出两个数，问两点之间的最短距离

          做法：蛇形矩阵 + bfs + 优先队列

**/

#include <iostream>

#include <stdio.h>

#include <cmath>

#include <algorithm>

#include <string.h>

#include <queue>

#define maxn 40000

using namespace std;

int mmap[][];

int a[][];

int vis[][];

int dx[] = {,,-,};

int dy[] = {,-,,};

int n,m;

bool num[maxn];

void is_prime()

{

    int tot = ;

    memset(num,false,sizeof(num));

    num[] = true;

    for(long long  i=; i<maxn; i++)

    {

        if(!num[i])

        {

            for(long long j=i*i; j<=maxn; j+=i)

            {

                num[j] = true;

            }

        }

    }

}

struct Node

{

    int x;

    int y;

    int step;

    Node() {}

    Node(int _x,int _y,int _step)

    {

        x = ;

        y = ;

        step =;

    }

} start,endd;

struct cmp

{

    bool operator () (const Node &a,const Node &b)

    {

        return a.step>b.step;

    }

};

int check(int x,int y)

{

    if(x>= && x < && y >=  && y <  && num[a[x][y]] == true&& !vis[x][y]) return ;

    return ;

}

priority_queue<Node,vector<Node>,cmp >que;

bool bfs()

{

    memset(vis,,sizeof(vis));

    Node tmp,now;

    while(!que.empty()) que.pop();

    que.push(start);

    vis[start.x][start.y] = ;

    start.step = ;

    while(!que.empty())

    {

        now = que.top();

        que.pop();

        //cout<<now.x<<"   "<<now.y<<"   "<<now.step<<endl;

        if(now.x == endd.x && now.y == endd.y)

        {

            endd.step = now.step;

            return true;

        }

        for(int i=; i<; i++)

        {

            tmp.x = now.x + dx[i];

            tmp.y = now.y + dy[i];

            tmp.step = now.step + ;

            if(check(tmp.x,tmp.y))

            {

                vis[tmp.x][tmp.y] = ;

                que.push(tmp);

            }

        }

    }

    return false;

}

void init()

{

    int x = ;

    int y = ;

    int nn = ;

    int num=a[][]=;

    while(num>)

    {

        while((y+)<nn&&!a[x][y+])  a[x][++y]= --num;

        while((x+)<nn&&!a[x+][y])  a[++x][y]= --num;

        while((y-)>=&&!a[x][y-]) a[x][--y]= --num;

        while((x-)>=&&!a[x-][y]) a[--x][y]= --num;

    }

}

int main()

{

    //freopen("in.txt","r",stdin);

    init();

    is_prime();

    int Case = ;

    while(~scanf("%d %d",&n,&m))

    {

        for(int i=; i<; i++)

        {

            for(int j=; j<; j++)

            {

                if(a[i][j] == n)

                {

                    start.x = i;

                    start.y = j;

                }

                if(a[i][j] == m)

                {

                    endd.x= i;

                    endd.y = j;

                }

            }

        }

        bool prime = false;

        prime = bfs();

        printf("Case %d: ",Case++);

        if(prime) printf("%d\n",endd.step);

        else printf("impossible\n");

    }

    return ;

}