典型的bfs，直接暴力枚举四位上的数字。

#include<iostream>
#include<cstring>
#include<cstdio>
#include<queue>
#include<map>
#include<set>
#include<cmath> 
using namespace std;
struct node{
	int x[4];
	int step;
}no,de;
queue<node>q;
int vis[10008],ans,a,b;
bool is_prime(int num){
	for(int i=2;i<=sqrt(num);i++){
		if(num%i==0) return 0;
	}
	return 1;
}
int write(int *a){
	return a[0]*1000+a[1]*100+a[2]*10+a[3];
}
void bfs(){
	while(!q.empty())	q.pop();
		memset(vis,0,sizeof(vis));
		ans=0;
		no.step=0;
		for(int i=0;i<4;i++){
			no.x[3-i]=a%10;
			a/=10;
		}
		q.push(no);
		vis[write(no.x)]=1;
	while(!q.empty()){
		node st=q.front();
		q.pop();
		if(write(st.x)==b){
			printf("%d\n",st.step);
			return ;
		}
		for(int i=0;i<4;i++){
			de=st;
			for(int j=0;j<10;j++){
				if(i==0&&j==0) continue;
				de.x[i]=j;
				if(!vis[write(de.x)]&&is_prime(write(de.x))){
					vis[write(de.x)]=1;
					de.step=st.step+1;
					q.push(de);
				}
			}
		}
	}
	printf("Impossible\n");
}
int main(){
	int t;
	scanf("%d",&t);
	while(t--){
		scanf("%d%d",&a,&b);
		bfs();
	}
	return 0;
}