题目： code：

#include<bits/stdc++.h>
#define ll long long
using namespace std;
struct node{
	int x,y;
};
int n,m,sum;
char zm1[30]={'0','A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'};
string sz[30]={"0","1","2","3","4","5","6","7","8","9","10","11","12","13","14","15","16","17","18","19","20","21","22","23","24","25","26"};
char zm[30];//用来装倒序字母，到时候会生成
string ans;
bool vis[30][30];
node off[8]={{2,-1},{2,1},{1,-2},{1,2},{-1,-2},{-1,2},{-2,-1},{-2,1}};//按照字典序排列的方向向量
void dfs(int sx,int sy){
	sum++;
	vis[sx][sy]=1;
	ans+=zm[sx];
	ans+=sz[sy];
	for(int i=0;i<8;i++){
		int x=sx+off[i].x;
		int y=sy+off[i].y;
		if(x>=1&&x<=n&&y>=1&y<=m&&!vis[x][y]){
			dfs(x,y);
		}
	}
}
int main(){
	cin>>m>>n;
	for(int i=1;i<=n;i++){
		zm[i]=zm1[n-i+1];
	}
	dfs(n,1);
	if(sum==n*m) cout<<ans;
	else cout<<"impossible";
	return 0;
}