#include <cstdio>
#include <cmath>
#include <cstring>
#include <cstdlib>
#include <climits>
#include <ctime>
#include <queue>
#include <stack>
#include <algorithm>
#include <list>
#include <vector>
#include <set>
#include <map>
#include <iostream>
#include <deque>
#include <complex>
#include <string>
#include <iomanip>
#include <sstream>
#include <bitset>
#include <valarray>
#include <iterator>
using namespace std;
typedef long long int ll;
typedef unsigned int uint;
typedef unsigned char uchar;
typedef unsigned long long ull;
typedef pair<int, int> pii;
typedef vector<int> vi;

#define REP(i,x) for(int i=0;i<(int)(x);i++)
#define FOR(i,c) for(__typeof((c).begin())i=(c).begin();i!=(c).end();i++)
#define RREP(i,x) for(int i=((int)(x)-1);i>=0;i--)
#define RFOR(i,c) for(__typeof((c).rbegin())i=(c).rbegin();i!=(c).rend();i++)
#define ALL(container) container.begin(), container.end()
#define RALL(container) container.rbegin(), container.rend()
#define SZ(container) ((int)container.size())
#define mp(a,b) make_pair(a, b)
#define UNIQUE(v) v.erase( unique(v.begin(), v.end()), v.end() );

template<class T> bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a=b; return 1; } return 0; }
template<class T> ostream& operator<<(ostream &os, const vector<T> &t) {
os<<"["; FOR(it,t) {if(it!=t.begin()) os<<","; os<<*it;} os<<"]"; return os;
}
template<class T> ostream& operator<<(ostream &os, const set<T> &t) {
os<<"{"; FOR(it,t) {if(it!=t.begin()) os<<","; os<<*it;} os<<"}"; return os;
}
template<class S, class T> ostream& operator<<(ostream &os, const pair<S,T> &t) { return os<<"("<<t.first<<","<<t.second<<")";}

const int INF = 1<<28;
const double EPS = 1e-8;
const int MOD = 1000000007;


int n, a, s, g;

typedef pair<string, int> Edge;
typedef vector< vector<Edge> > Graph;
int gg[41][41];
Graph G;

int main(){
	while(cin >> n >> a >> s >> g, n){
		Graph G(n);
		memset(gg, 0, sizeof(gg));
		REP(i, a){
			int u, v;
			string sp;
			cin >> u >> v >> sp;
			gg[u][v] = 1;
			G[u].emplace_back(sp, v);
		}
		REP(i, n){
			sort(ALL(G[i]));
			gg[i][i] = 1;
		}
		REP(k, n)REP(i, n)REP(j, n) gg[i][j] |= gg[i][k] & gg[k][j];
		
		set<Edge> pq;
		pq.insert(Edge("", s));
		Edge S;
		while(!pq.empty()){
			S = *(pq.begin());pq.erase(pq.begin());
			if(S.second == g || S.first.size() > 6*n) break;
			FOR(e, G[S.second]){
				if(gg[e->second][g]) pq.insert(Edge(S.first + e->first, e->second));
			}
		}
		cout << (S.second == g ? S.first : "NO") << endl;
	}
	return 0;
}