// ACM-ICPC国内予選2010 E. 最強の呪文

#include <iostream>
#include <string>
#include <vector>
#include <queue>

using namespace std;

typedef vector< vector< pair<int, string> > > Graph;

class Node{
public:
	int pos; string str; int prev;
	Node(int pos, string str, int prev) : pos(pos), str(str), prev(prev) {}
	bool operator < (const Node &nd) const { return str > nd.str; }
};

int main(){
	int n, a, s, g;
	int prev[480][40];
	string str[480][40];
	while(cin >> n >> a >> s >> g, n){
		Graph graph(n);
		for(int i=0;i<a;i++){
			int x, y; string lab; cin >> x >> y >> lab;
			graph[x].push_back(make_pair(y,lab));
		}
		memset(prev, -1, sizeof(prev));
		for(int i=0;i<12*n;i++)
			for(int j=0;j<n;j++) str[i][j] = "";
		priority_queue<Node> qu; qu.push(Node(s, "", -2));
		while(!qu.empty()){
			Node nd = qu.top(); qu.pop();
			int pos = nd.pos, size = nd.str.size();
			if(prev[size][pos]!=-1) continue;
			prev[size][pos] = nd.prev;
			str[size][pos] = nd.str;
			for(int i=0;i<graph[pos].size();i++){
				int npos = graph[pos][i].first;
				if(size + graph[pos][i].second.size() >= 12*n) continue;
				string nstr = nd.str + graph[pos][i].second;
				if(str[nstr.size()][npos]==""||nstr<str[nstr.size()][npos]){
					str[nstr.size()][npos] = nstr;
					qu.push(Node(npos, nstr, size*n+pos));
				}
			}
		}
		int minpos = -1;
		string ans = "";
		for(int i=0;i<12*n;i++){
			if(str[i][g]=="") continue;
			if(ans == ""||ans > str[i][g]){
				ans = str[i][g];
				minpos = i;
			}
		}
		bool flag = (minpos != -1);
		bool visit[40];
		memset(visit, false, sizeof(visit));
		int cur = minpos*n+g;
		while(flag&&cur!=-2){
			if(visit[cur%n]) flag = false;
			visit[cur%n] = true;
			cur = prev[cur/n][cur%n];
		}
		cout << (flag ? ans : "NO") << endl;
	}
}