#include<bits/stdc++.h>
#define int long long
using namespace std;
struct node{int x,y,step;};
int T,n,m,flag; 
char a[2010][2010],way[2010][2010];
bool vis[2010][2010];
int dx[]={1,-1,0,0},dy[]={0,0,1,-1};
signed main(){
	cin>>T;
	while(T--){
		cin>>n>>m;
		for(int i=1;i<=n;i++){
			for(int j=1;j<=m;j++){
				cin>>a[i][j];
				vis[i][j]=false;
				way[i][j]=' ';
			}
		}
		flag=0;
		queue<node> q;
		q.push(node{1,1,0});
		vis[1][1]=1;
		while(!q.empty()){
			node t=q.front();
			q.pop();
			if(t.x==n&&t.y==m){
				cout<<t.step<<endl;
				flag=1;
				break;
			}
			for(int i=0;i<4;i++){
				int tx=t.x+dx[i],ty=t.y+dy[i];
				if(tx>=1&&tx<=n&&ty>=1&&ty<=m&&!vis[tx][ty]&&a[tx][ty]!=a[t.x][t.y]){
					if(tx==t.x+1&&ty==t.y)way[tx][ty]='D';
					if(tx==t.x-1&&ty==t.y)way[tx][ty]='U';
					if(tx==t.x&&ty==t.y+1)way[tx][ty]='R';
					if(tx==t.x&&ty==t.y-1)way[tx][ty]='L';
					q.push(node{tx,ty,t.step+1});
				}
			}
		}
		if(flag==0){
			cout<<-1<<endl;
			continue;
		}
		/*for(int i=1;i<=n;i++){
			for(int j=1;j<=m;j++){
				cout<<way[i][j]<<" ";
			}
			cout<<endl;
		}*/
		stack<char> s;
		for(int i=n,j=m;;){
			if(i==1&&j==1)break;
			s.push(way[i][j]);
			if(way[i][j]=='D')i=i-1;
			else if(way[i][j]=='U')i=i+1;
			else if(way[i][j]=='R')j=j-1;
			else if(way[i][j]=='l')j=j+1;
		}
		while(!s.empty()){cout<<s.top();s.pop();}		
		cout<<endl;
	}
	return 0;
}