#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <climits>
#include <cassert>
#include <iostream>
#include <iomanip>
#include <sstream>
#include <algorithm>
#include <numeric>
#include <complex>
#include <stack>
#include <queue>
#include <list>
#include <set>
#include <map>
#include <bitset>
#include <functional>
#include <iterator>

using namespace std;

#define dump(n) cout<<"# "<<#n<<'='<<(n)<<endl
#define repi(i,a,b) for(int i=int(a);i<int(b);i++)
#define peri(i,a,b) for(int i=int(b);i-->int(a);)
#define rep(i,n) repi(i,0,n)
#define per(i,n) peri(i,0,n)
#define iter(c) __typeof__((c).begin())
#define foreach(i,c) for(iter(c) i=(c).begin();i!=(c).end();++i)
#define all(c) (c).begin(),(c).end()
#define mp make_pair

typedef unsigned int uint;
typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<double> vd;
typedef vector<vd> vvd;
typedef vector<string> vs;
typedef vector<vs> vvs;
typedef pair<int,int> pii;

const int INFTY=1<<29;
const double EPS=1e-9;

template<typename T1,typename T2>
ostream& operator<<(ostream& os,const pair<T1,T2>& p){
	return os<<'('<<p.first<<','<<p.second<<')';
}
template<typename T>
ostream& operator<<(ostream& os,const vector<T>& a){
	os<<'[';
	rep(i,a.size()) os<<(i?" ":"")<<a[i];
	return os<<']';
}

struct Edge{
	int src,dst;
	string label;
	Edge(){}
	Edge(int s,int d,string l):src(s),dst(d),label(l){}
	bool operator<(const Edge& e)const{return label<e.label;}
	bool operator>(const Edge& e)const{return label>e.label;}
};
typedef vector<vector<Edge>> Graph;

int main()
{
	for(int n,m,s,t;cin>>n>>m>>s>>t,n|m|s|t;){
		Graph g(n);
		rep(_,m){
			int x,y; string l; cin>>x>>y>>l;
			g[y].emplace_back(y,x,l);
		}
		
		vs res(n,"~");
		res[t]="";
		rep(_,6*n) rep(i,n) if(res[i]!="~") for(Edge e:g[i])
			res[e.dst]=min(res[e.dst],e.label+res[e.src]);
		
		if(res[s]=="~" || res[s].size()>=6*n)
			cout<<"NO"<<endl;
		else
			cout<<res[s]<<endl;
	}
	
	return 0;
}