#include <bits/stdc++.h>
using namespace std;
/*{{{*/  //template
#define rep(i,n) for(int i=0;i<n;i++)
constexpr int INF = numeric_limits<int>::max()/2;
constexpr long long LINF = numeric_limits<long long>::max()/3;
#define mp make_pair
#define pb push_back
#define eb emplace_back
#define fi first
#define se second
#define all(v) (v).begin(),(v).end()
#define sz(x) (int)(x).size()
#define debug(x) cerr<<#x<<":"<<x<<endl
#define debug2(x,y) cerr<<#x<<","<<#y":"<<x<<","<<y<<endl
//struct fin{ fin(){ cin.tie(0); ios::sync_with_stdio(false); } } fin_;
struct Double{ double d; explicit Double(double x) : d(x){} };
ostream& operator<<(ostream& os,const Double x){ os << fixed << setprecision(20) << x.d; return os; }
template<typename T> ostream& operator<<(ostream& os,const vector<T>& vec){ os << "["; for(const auto& v : vec){ os << v << ","; } os << "]"; return os; }
template<typename T,typename U> ostream& operator<<(ostream& os,const pair<T,U>& p){ os << "(" << p.first << ","<< p.second <<")"; return os; }
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> pii;
typedef vector<int> vi;
typedef vector<vi> vvi;
ll gcd(ll a,ll b){ if(b==0) return a; else return gcd(b,a%b); }
//constexpr double eps = 1e-14; 
constexpr double eps = 1e-10; 
constexpr ll mod = 1e9+7;
const int dx[]={1,0,-1,0} ,dy[] = {0,1,0,-1};
/*}}}*/

struct edge{
    int from,to;
    string cost;
    edge(){}
    edge(int a,int b,string s) : from(a),to(b),cost(s){}
};


int V,E,s,g;

void solve(){
    vector<edge> es(E);
    vector<edge> rev(E);
    rep(i,E){
        cin>>es[i].from>>es[i].to>>es[i].cost;
        rev[i] = edge(es[i].to,es[i].from,es[i].cost);
    }

    vector<string> d(V,"NO");
    d[g] = "";
    int cnt=0;
    while(true){
        bool update = false;
        for(edge e : rev){
            if(d[e.from] == "NO") continue;
            if(d[e.to] == "NO" || d[e.to] > e.cost + d[e.from]){
                d[e.to] = e.cost + d[e.from];
                update = true;
            }
        }
        if(!update) break;
        if(++cnt==300) break;
    }
    if(d[s].size()>=6*(V-1)){
        cout << "NO" << endl;
        return;
    }
    cout << d[s] << endl;
}

int main(){
    while(true){
        cin >> V >> E >> s >> g;
        //cout << V << " " << E <<" " << s << " " << g << endl;
        if(V==0 && E==0 && s==0 && g==0) break;
        solve();
    }
}