#include <iostream>
#include <cstdio>
#include <vector>
#include <list>
#include <complex>
#include <algorithm>
#include <set>
#include <map>
#include <queue>
#include <string>
#include <cstring>
#include <stack>
#include <cmath>
#include <iomanip>
#include <sstream>
#include <cassert>
#include <numeric>
using namespace std;
typedef long long ll;
typedef ll li;
typedef pair<int,int> PI;
#define EPS (1e-6)
#define rep(i,n) for(int i=0;i<(int)(n);++i)
#define REP(i, n) rep (i, n)
#define F first
#define S second
#define mp(a,b) make_pair(a,b)
#define pb(a) push_back(a)
#define min3(a,b,c) min((a),min((b),(c)))
#define min4(a,b,c,d) min((a),min3((b),(c),(d)))
#define SZ(a) (int)((a).size())
#define ALL(a) a.begin(),a.end()
#define FLL(a,b) memset((a),b,sizeof(a))
#define CLR(a) memset((a),0,sizeof(a))
#define FOR(it,a) for(__typeof(a.begin())it=a.begin();it!=a.end();++it)
template<typename T,typename U> ostream& operator<< (ostream& out, const pair<T,U>& val){return out << "(" << val.F << ", " << val.S << ")";}
template<class T> ostream& operator<< (ostream& out, const vector<T>& val){out << "{";rep(i,SZ(val)) out << (i?", ":"") << val[i];return out << "}";}
typedef double FP;
typedef complex<FP> pt;
typedef pt P;
typedef pair<pt,pt> line;
FP dot(P a,P b){return real(conj(a)*b);}
FP crs(P a,P b){return imag(conj(a)*b);}
P ortho(P a){return conj(P(imag(a),real(a)));}
P ortho(line a){return ortho(a.S-a.F);}
P crspt(P a,P b,P c,P d){b-=a,d-=c;return a+b*crs(d,c-a)/crs(d,b);}
P crspt(line a,line b){return crspt(a.F,a.S,b.F,b.S);}
bool onl(P a1,P a2,P b){return abs(b-a1)+abs(b-a2)<abs(a1-a2)+EPS;}
bool onl(line a,P b){return onl(a.F,a.S,b);}
bool iscrs(line a,line b){P c=crspt(a,b);return onl(a,c)&&onl(b,c);}
void pkuassert(bool t){t=1/t;};
int dx[]={0,1,0,-1,1,1,-1,-1};
int dy[]={1,0,-1,0,-1,1,1,-1};
enum{TOP,BTM,LFT,RGT,FRT,BCK};
int dxdy2ce[]={RGT,FRT,LFT,BCK};
template<class T> T shift(T a,int b,int c,int d,int e){
  __typeof(a[0])t=a[b];
  a[b]=a[c];a[c]=a[d];a[d]=a[e];a[e]=t;return a;}
template<class T> T rgt(T a){return shift(a,TOP,LFT,BTM,RGT);}
template<class T> T lft(T a){return shift(a,TOP,RGT,BTM,LFT);}
template<class T> T frt(T a){return shift(a,TOP,BCK,BTM,FRT);}
template<class T> T bck(T a){return shift(a,TOP,FRT,BTM,BCK);}
line mkl(P a,P v){return line(a,a+v);}
FP lpdist(line a,P b){
  return abs(b-crspt(a,mkl(b,ortho(a))));
}

FP spdist(line a,P b){
  P c(crspt(a,mkl(b,ortho(a))));
  return onl(a,c)?abs(b-c):min(abs(a.F-b),abs(a.S-b));
}

FP ssdist(line a,line b){
  return
    min4(spdist(a,b.F),spdist(a,b.S),
         spdist(b,a.F),spdist(b,a.S));
}


int n,a,s,g;
vector<pair<int,string> > G[40];
bool can[40][40];
string vis[40][300];
void solve(){
  rep(i,n) G[i].clear();
  rep(i,n)rep(j,300) vis[i][j]=string()+char(max('z','Z')+1);
  CLR(can);
  rep(i,a){
    int x,y;
    string l;
    cin >> x >> y >> l;
    G[x].pb(mp(y,l));
    can[x][y]=1;
  }
  rep(i,n) can[i][i]=1;
  rep(k,n)rep(i,n)rep(j,n)
    can[i][j]|=can[i][k]&can[k][j];
  set<pair<string,int> > q;
  q.insert(mp("",s));
  while(!q.empty()){
    string cc=q.begin()->F;
    int cv=q.begin()->S;
    q.erase(q.begin());
    if(SZ(cc)>300){
      cout << "NO" << endl;
      return;
    }
    if(vis[cv][SZ(cc)]<=cc) continue;
    //cout << cv << ' ' << cc << endl;
    if(cv==g){
      cout << cc << endl;
      return;
    }
    vis[cv][SZ(cc)]=cc;
    FOR(it,G[cv])
      if(can[it->F][g])
        q.insert(mp(cc+it->S,it->F));
  }

  cout << "NO" << endl;
}

int main(int argc, char *argv[])
{
  while(cin >> n >> a >> s >> g && n) solve();
  return 0;
}

Measured memory_bytes: 17412