#include<deque>
#include<queue>
#include<vector>
#include<algorithm>
#include<iostream>
#include<set>
#include<cmath>
#include<tuple>
#include<string>
//#include<chrono>
#include<functional>
#include<iterator>
#include<random>
#include<unordered_set>
#include<array>
#include<map>
#include<iomanip>
#include<assert.h>
#include<bitset>
#include<stack>
#include<memory>
using namespace std;
//using namespace std::chrono;
typedef long long int llint;
typedef double lldo;
#define mp make_pair
#define mt make_tuple
#define pub push_back
#define puf push_front
#define pob pop_back
#define pof pop_front
#define fir first
#define sec second
#define res resize
#define ins insert
#define era erase

/*
cout<<fixed<<setprecision(20);
cin.tie(0);
ios::sync_with_stdio(false);
*/
const llint mod=1000000007;
const llint big=2.19e15+1;
const long double pai=3.141592653589793238462643383279502884197;
const long double eps=1e-7;
template <class T,class U>bool mineq(T& a,U b){if(a>b){a=b;return true;}return false;}
template <class T,class U>bool maxeq(T& a,U b){if(a<b){a=b;return true;}return false;}
llint gcd(llint a,llint b){if(a%b==0){return b;}else return gcd(b,a%b);}
llint lcm(llint a,llint b){if(a==0){return b;}return a/gcd(a,b)*b;}
template<class T> void SO(T& ve){sort(ve.begin(),ve.end());}
template<class T> void REV(T& ve){reverse(ve.begin(),ve.end());}
template<class T>llint LBI(vector<T>&ar,T in){return lower_bound(ar.begin(),ar.end(),in)-ar.begin();}
template<class T>llint UBI(vector<T>&ar,T in){return upper_bound(ar.begin(),ar.end(),in)-ar.begin();}
bool solve(void){
	//逆順にたどることで貪欲が可能になります。私って天才?
	int n,m,sta,gol,i,j,k;cin>>n>>m>>sta>>gol;
	if(n==0){return false;}
	vector<string>magic(n,"~");// ~ is weakest
	magic[gol]="#";
	vector<tuple<int,int,string>>road(m);
	for(i=0;i<m;i++){
		int x,y;cin>>x>>y;
		string lab;cin>>lab;
		road[i]=mt(x,y,lab);
	}
	for(i=0;i<n*10;i++){
		for(j=0;j<m;j++){
			int x,y;string lab;
			tie(x,y,lab)=road[j];
			if(magic[y].back()=='~'){continue;}
			mineq(magic[x],lab+magic[y]);
		}
	}
	string ansA=magic[sta];
	for(i=0;i<n*10;i++){
		for(j=0;j<m;j++){
			int x,y;string lab;
			tie(x,y,lab)=road[j];
			if(magic[y].back()=='~'){continue;}
			mineq(magic[x],lab+magic[y]);
		}
	}
	string ansB=magic[sta];
	if(ansA.back()=='~'||ansB<ansA){cout<<"NO"<<endl;}
	else{ansA.pob();cout<<ansA<<endl;}
	return true;
}

int main(void){
	while(solve()){}
	return 0;
}

