#include <iostream>
#include <sstream>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <ctime>
#include <cstring>
#include <string>
#include <vector>
#include <stack>
#include <queue>
#include <deque>
#include <map>
#include <set>
#include <bitset>
#include <numeric>
#include <utility>
#include <iomanip>
#include <algorithm>
#include <functional>
using namespace std;

typedef long long ll;
typedef vector<int> vint;
typedef vector<ll> vll;
typedef pair<int,int> pint;

#define DE 1
#define FI first
#define SE second
#define PB push_back
#define MP make_pair
#define ALL(s) (s).begin(),(s).end()
#define REP(i,n) for (int i = 0; i < (int)(n); ++i)
#define EACH(i,s) for (typeof((s).begin()) i = (s).begin(); i != (s).end(); ++i)
#define COUT(x) cout<<#x<<" = "<<(x)<<" (L"<<__LINE__<<")"<<endl

template<class T1, class T2> ostream& operator<<(ostream &s, pair<T1,T2> P){return s<<'<'<<P.first<<", "<<P.second<<'>';}
template<class T> ostream& operator<<(ostream &s, vector<T> P) {s<<"{ ";for(int i=0;i<P.size();++i){if(i>0)s<<", ";s<<P[i];}return s<<" }"<<endl;}
template<class T1, class T2> ostream& operator<<(ostream &s, map<T1,T2> P) {s<<"{ ";for(typeof(P.begin()) it=P.begin();it!=P.end();++it){if(it!=P.begin())s<<", ";s<<'<'<<it->first<<"->"<<it->second<<'>';}return s<<" }"<<endl;}





template<class T> struct Edge {
    int from, to;
    T cost;
    Edge(int f, int t, T c) : from(f), to(t), cost(c) {}
    friend bool operator < (const Edge &e1, const Edge &e2) {return e1.cost < e2.cost;}
    friend ostream& operator << (ostream& s, const Edge& E) {s << E.from << "->" << E.to << '(' << E.cost << ')';return s;}
};

const int MAX_V = 45;
template<class T> struct Graph {
    static const int MAX_V_ = ::MAX_V;
    int V,E;
    vector<Edge<T> > list[MAX_V_];
    
    Graph(int n = 0) : V(n), E(0) {}
    void init(int n) {V = n; E = 0; for (int i = 0; i < MAX_V_; ++i) list[i].clear();}
    inline vector<Edge<T> >& operator [] (int i) {return list[i];}
    friend ostream& operator << (ostream& s, const Graph& G) {for (int i = 0; i < G.V; ++i) {s << i << " : " << G.list[i];}return s;}
    
    void direct(int f, int t, T c) {
        ++E;
        list[f].push_back(Edge<T>(f,t,c));
    }
};

Graph<string> G;

string INF = "{}";
string dp[45][500];

int main() {
    //freopen( "/Users/macuser/Documents/Programming/Contest/input.in", "r", stdin );
    
    int n, a, s, g;
    while (cin >> n >> a >> s >> g) {
        if (n == 0) break;
        
        G.init(n);
        for (int i = 0; i < a; ++i) {
            int from, to; string cost;
            cin >> from >> to >> cost;
            G.direct(from, to, cost);
        }
        
        for (int i = 0; i < 45; ++i) for (int j = 0; j < 500; ++j) dp[i][j] = INF;
        dp[s][0] = "";
        
        priority_queue< pair<string,pint>, vector<pair<string,pint> >, greater<pair<string,pint> > > que;
        que.push(MP("",MP(s,0)));
        
        while (!que.empty()) {
            string str = que.top().FI;
            int node = que.top().SE.FI, len = que.top().SE.SE;
            
            que.pop();
            if (str > dp[node][len]) continue;
            
            for (int i = 0; i < G[node].size(); ++i) {
                string nstr = str + G[node][i].cost;
                int nnode = G[node][i].to;
                int nlen = nstr.size();
                
                if (nlen >= 500) continue;
                if (nstr < dp[nnode][nlen]) {
                    dp[nnode][nlen] = nstr;
                    que.push(MP(nstr,MP(nnode,nlen)));
                }
            }
        }
        
        string res = INF;
        int minind = -1;
        for (int i = 0; i < 500; ++i) {
            if (res > dp[g][i]) {
                res = dp[g][i];
                minind = i;
            }
        }
        
        if (res < INF && minind < 250) cout << res << endl;
        else cout << "NO" << endl;
    }
    
    
    return 0;
}


Measured memory_bytes: 22748