素数路(prime)

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <algorithm>
#include <climits>
#include <cstring>
#include <string>
#include <set>
#include <map>
#include <queue>
#include <stack>
#include <vector>
#include <list>
#include <ext/rope>
#define rep(i,m,n) for(i=m;i<=n;i++)
#define rsp(it,s) for(set<int>::iterator it=s.begin();it!=s.end();it++)
#define vi vector<int>
#define pii pair<int,int>
#define mod 1000000007
#define inf 0x3f3f3f3f
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define ll long long
#define pi acos(-1.0)
const int maxn=1e5+10;
const int dis[4][2]={{0,1},{-1,0},{0,-1},{1,0}};
using namespace std;
using namespace __gnu_cxx;
ll gcd(ll p,ll q){return q==0?p:gcd(q,p%q);}
ll qpow(ll p,ll q){ll f=1;while(q){if(q&1)f=f*p;p=p*p;q>>=1;}return f;}
int n,m,all,a[maxn],vis[maxn];
bool sushu(int p)
{
    if(p<=1)return false;
    else if(p==2)return true;
    for(int i=3;i<p;i+=2)if(p%i==0)return false;
    return true;
}
void dfs()
{
    queue<int>p;p.push(n);vis[n]=1;
    while(!p.empty())
    {
        int u=p.front(),v;
        p.pop();
        if(u==m)return;
        for(int i=0;i<=9;i++)
        {
            v=u-u%10+i;
            if(!vis[v]&&a[v])p.push(v),vis[v]=vis[u]+1;
        }
        for(int i=0;i<=9;i++)
        {
            v=u-u/10%10*10+i*10;
            if(!vis[v]&&a[v])p.push(v),vis[v]=vis[u]+1;
        }
        for(int i=0;i<=9;i++)
        {
            v=u-u/100%10*100+i*100;
            if(!vis[v]&&a[v])p.push(v),vis[v]=vis[u]+1;
        }
        for(int i=0;i<=9;i++)
        {
            v=u-u/1000*1000+i*1000;
            if(!vis[v]&&a[v])p.push(v),vis[v]=vis[u]+1;
        }
    }
}
int main()
{
    int i,j,k,t;
    scanf("%d%d",&n,&m);
    for(int i=1000;i<=9999;i++)
        if(sushu(i))a[i]=1;
    dfs();
    if(vis[m])printf("%d\n",vis[m]-1);
    else puts("Impossible");
    //system ("pause");
    return 0;
}