#include<iostream>
#include<string>
#include<cstdio>
#include<vector>
#include<cmath>
#include<algorithm>
#include<functional>
#include<iomanip>
#include<queue>
#include<ciso646>
#include<random>
#include<map>
#include<set>
#include<bitset>
#include<stack>
#include<unordered_map>
#include<utility>
#include<cassert>
#include<complex>
#include<numeric>
using namespace std;

//#define int long long
typedef long long ll;

typedef unsigned long long ul;
typedef unsigned int ui;
const ll mod = 1000000007;
const ll INF = (1e+18) + 7;
typedef pair<int, int>P;
#define stop char nyaa;cin>>nyaa;
#define rep(i,n) for(int i=0;i<n;i++)
#define per(i,n) for(int i=n-1;i>=0;i--)
#define Rep(i,sta,n) for(int i=sta;i<n;i++)
#define rep1(i,n) for(int i=1;i<=n;i++)
#define per1(i,n) for(int i=n;i>=1;i--)
#define Rep1(i,sta,n) for(int i=sta;i<=n;i++)
#define all(v) (v).begin(),(v).end()
typedef pair<ll, ll> LP;
typedef long double ld;
typedef pair<ld, ld> LDP;
const ld eps = 1e-6;
const ld pi = acos(-1.0);
//typedef vector<vector<ll>> mat;
typedef vector<int> vec;

ll mod_pow(ll a, ll n) {
	ll res = 1;
	while (n) {
		if (n & 1)res = res * a%mod;
		a = a * a%mod; n >>= 1;
	}
	return res;
}

struct modint {
	ll n;
	modint() :n(0) { ; }
	modint(ll m) :n(m) {
		if (n >= mod)n %= mod;
		else if (n < 0)n = (n%mod + mod) % mod;
	}
	operator int() { return n; }
};
bool operator==(modint a, modint b) { return a.n == b.n; }
modint operator+=(modint &a, modint b) { a.n += b.n; if (a.n >= mod)a.n -= mod; return a; }
modint operator-=(modint &a, modint b) { a.n -= b.n; if (a.n < 0)a.n += mod; return a; }
modint operator*=(modint &a, modint b) { a.n = ((ll)a.n*b.n) % mod; return a; }
modint operator+(modint a, modint b) { return a += b; }
modint operator-(modint a, modint b) { return a -= b; }
modint operator*(modint a, modint b) { return a *= b; }
modint operator^(modint a, int n) {
	if (n == 0)return modint(1);
	modint res = (a*a) ^ (n / 2);
	if (n % 2)res = res * a;
	return res;
}

ll inv(ll a, ll p) {
	return (a == 1 ? 1 : (1 - p * inv(p%a, a)) / a + p);
}
modint operator/(modint a, modint b) { return a * modint(inv(b, mod)); }

//const int max_n = 1 << 22;
//modint fact[max_n], factinv[max_n];
//void init_f() {
//	fact[0] = modint(1);
//	for (int i = 0; i < max_n - 1; i++) {
//		fact[i + 1] = fact[i] * modint(i + 1);
//	}
//	factinv[max_n - 1] = modint(1) / fact[max_n - 1];
//	for (int i = max_n - 2; i >= 0; i--) {
//		factinv[i] = factinv[i + 1] * modint(i + 1);
//	}
//}
//modint comb(int a, int b) {
//	if (a < 0 || b < 0 || a < b)return 0;
//	return fact[a] * factinv[b] * factinv[a - b];
//}

struct edge {
	int to,cost;
};
int n, m, s, t;

vector<edge> G[3000];
void solve() {
	rep(i, 3000) {
		G[i].clear();
	}
	int tmp = n;
	rep(i, m) {
		int a, b; string c; cin >> a >> b >> c;
		if (c.size() == 1) {
			G[a].push_back({ b,c[0] - 'a' });
		}
		else {
			int len = c.size();
			G[a].push_back({ tmp,c[0] - 'a' });
			G[tmp + len - 2].push_back({ b,c.back() - 'a' });
			rep(j, len - 2) {
				G[tmp + j].push_back({ tmp + j + 1,c[j + 1] - 'a' });
			}
			tmp += len - 1;
		}
	}
	vector<vector<int>> rG(tmp);
	rep(i, tmp)for (edge e : G[i]) {
		rG[e.to].push_back(i);
	}
	vector<bool> exi(tmp, false);
	exi[t] = true;
	queue<int> q; q.push(t);
	while (!q.empty()) {
		int id = q.front(); q.pop();
		for (int to : rG[id]) {
			if (!exi[to]) {
				exi[to] = true;
				q.push(to);
			}
		}
	}
	if (!exi[s]) {
		cout << "NO" << endl; return;
	}
	vector<int> mi(tmp,mod);
	rep(i, tmp)for (edge e : G[i]) {
		if (exi[e.to]) {
			mi[i] = min(mi[i], e.cost);
		}
	}
	string ans;
	vector<int> v;
	v.push_back(s);
	rep(i, 3000) {
		int z = mod;
		for (int id : v) {
			z = min(z, mi[id]);
		}
		ans.push_back('a' + z);
		vector<bool> nex(tmp, false);
		for (int id : v) {
			for (edge e : G[id]) {
				if (e.cost == z) {
					if (exi[e.to]) {
						nex[e.to] = true;
						if (e.to == t) {
							cout << ans << endl; return;
						}
					}
				}
			}
		}
		v.clear();
		rep(j, tmp) {
			if (nex[j])v.push_back(j);
		}
		if (v.empty())break;
	}
	cout << "NO" << endl;
}

signed main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	//cout << fixed << setprecision(12);
	//init_f();
	//int t; cin >> t; rep(i, t)solve();
	while (cin >>n>>m>>s>>t, n)solve();
	//solve();
	stop
		return 0;
}



Measured memory_bytes: 3368