#include <iostream>
#include <cstdio>
#include <iomanip>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <bitset>
#include <stack>
#include <utility>
#include <numeric>
#include <algorithm>
#include <functional>
#include <cctype>
#include <complex>
#include <string>
#include <sstream>
#include <fstream>
#include <cassert>
using namespace std;


//common

#define BR "\n"
#define ALL(c) (c).begin(),(c).end()
#define REP(i,n) for(int i=0;i<(int)(n);++i)
#define FOR(i,l,r) for(int i=(int)l;i<(int)(r);++i)
#define EACH(it,o) for(auto it = (o).begin(); it != (o).end(); ++it)
#define IN(l,v,r) ((l)<=(v) && (v)<(r))

//config
//#define NDEBUG

//debug
#ifdef NDEBUG
#define DUMP(x)
#define DUMPLN(x)
#define DEBUG(x)
#define DEBUGLN(x)
#define LINE()
#define LINELN()
#define CHECK(exp,act)
#define STOP(e)
#else
#define DUMP(x)  cerr << #x << " = " << (x)
#define DUMPLN(x)  DUMP(x) <<endl
#define DEBUG(x) DUMP(x) << LINE() << " " << __FILE__
#define DEBUGLN(x) DEBUG(x)<<endl
#define LINE()    cerr<< " (L" << __LINE__ << ")"
#define LINELN()    LINE()<<endl
#define CHECK(exp,act)  if(exp!=act){DUMPLN(exp);DEBUGLN(act);}
#define STOP(e)  CHECK(e,true);if(!(e)) exit(1);
#endif

template<class T> inline string toString(const vector<T>& x) {
	stringstream ss;
	REP(i,x.size()){
		if(i!=0)ss<<" ";
		ss<< x[i];
	}
	return ss.str();
}

template<class T> inline string toString(const vector<vector<T> >& map) {
	stringstream ss;
	REP(i,map.size()){
		if(i!=0)ss<<BR;
		ss<< toString(map[i]);
	}
	return ss.str();
}
template<class K,class V>  string toString(map<K,V>& x) {
	string res;stringstream ss;
	for(auto& p:x)ss<< p.first<<":" << p.second<<" ";
	return ss.str();
}

string BITtoString(int bit){
    stringstream ss;
    while(bit!=0){ss<<(bit%2);bit/=2;}
    string res=ss.str();reverse(ALL(res));
    return res;
}

template<typename T,typename V> inline T pmod(T v,V MOD){
	return (v%MOD+MOD)%MOD;
}
char tmp[1000];
#define nextInt(n) scanf("%d",&n)
#define nextLong(n) scanf("%lld",&n)
#define nextDouble(n) scanf("%lf",&n)  //I64d
#define nextChar(n) scanf("%c",&n)
#define nextString(n) scanf("%s",tmp);n=tmp

namespace EGraph{
	typedef string Cost;Cost CINF=string(1,'z'+1);
	struct Edge{
		int from,to;Cost cost;
		Edge(int from,int to,Cost cost)
		: from(from),to(to),cost(cost) {};
		bool operator<(Edge r) const{
			return cost<r.cost;
		}
		bool operator>(Edge r) const{
			return cost>r.cost;
		}
	};
	typedef vector<vector<Edge> >  Graph;
}
using namespace EGraph;

namespace ShortestPath{
    using namespace EGraph;

    struct Task{
        int prev,pos;Cost cost;
        Task(int prev,int pos,Cost cost)
            :prev(prev),pos(pos),cost(cost){};
        bool operator>(const Task& r) const{
            return cost>r.cost;
        }
    };
 
    // 最大も可
    // 負の閉路が判定可能
    //O(V*E)
    vector<Cost> bellmanFord(Graph g,int s,int t){
        const int V=g.size();
        vector<Cost> d(V,CINF);d[s]="";
        REP(k,6*V+10){
            REP(i,V)REP(j,g[i].size()){
                Edge& e=g[i][j];
                if(d[e.from]!=CINF &&(d[e.to]==CINF || d[e.to]>e.cost+d[e.from])){
                    d[e.to]=e.cost+d[e.from];
                    if(e.to==t && k>=V)d[e.to]=CINF;
                }
            }
        }
        return d;
    }
}
using namespace ShortestPath;
int n,a, s, t;

int main() {


	// ifstream cin("in");
	// ofstream cout( "out" );
	while (true) {
		cin >> n >> a >> s >> t;
		if (!n) break;
		
		Graph g(n);
		REP(i,a){
			int from,to;string lab;
			cin >> from >> to >>lab;
			g[to].push_back(Edge(to,from,lab));
		}

		string res=bellmanFord(g,t,s)[s];
		cout << (res==CINF?"NO":res) <<endl;
	}
}

Measured memory_bytes: 1716