#include <vector>
#include <iostream>
#include <utility>
#include <algorithm>
#include <string>
#include <deque>
#include <tuple>
#include <queue>
#include <stack>
#include <functional>
#include <cmath>
#include <iomanip>
#include <map>
#include <set>
#include <numeric>
#include <unordered_map>
#include <unordered_set>
#include <complex>
#include <iterator>
#include <array>
#include <memory>
#include <random>
#include <valarray>
//cin.sync_with_stdio(false);
//streambuf
using namespace std;
typedef long long ll;
typedef double ld;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef pair<double, double> pdd;
using vi = vector<int>;
using vll = vector<ll>;
using vpii = vector<pii>;
using vpll = vector<pll>;
using ti3 = tuple<int, int, int>;
using vti3 = vector<ti3>;
template<class T, int s>using va = vector<array<T, s>>;
template<class T, class T2> using umap = unordered_map<T, T2>;
template<class T> using uset = unordered_set<T>;
template<class T, class S> void cmin(T &a, const S &b) { if (a > b)a = b; }
template<class T, class S> void cmax(T &a, const S &b) { if (a < b)a = b; }
#define ALL(a) a.begin(),a.end()
#define rep(i,a) for(int i=0;i<a;i++)
#define rep1(i,a) for(int i=1;i<=a;i++)
#define rrep(i,a) for(int i=(a)-1;i>=0;i--)
#define rrep1(i,a) for(int i=a;i;i--)
#define repi(i,a,b) for(int i=a;i<b;i++)
//const ll mod = 1000000007;
template<class T>using heap = priority_queue<T, vector<T>, greater<T>>;
template<class T>using pque = priority_queue<T, vector<T>, function<T(T, T)>>;
template <class T>
inline void hash_combine(size_t & seed, const T & v) {
	hash<T> hasher;
	seed ^= hasher(v) + 0x9e3779b97f4a7c15 + (seed << 6) + (seed >> 2);
}
namespace std {
	template<typename S, typename T> struct hash<pair<S, T>> {
		inline size_t operator()(const pair<S, T> & v) const {
			size_t seed = 0;
			hash_combine(seed, v.first);
			hash_combine(seed, v.second);
			return seed;
		}
	};
	// Recursive template code derived from Matthieu M.
	template <class Tuple, size_t Index = std::tuple_size<Tuple>::value - 1>
	struct HashValueImpl {
		static void apply(size_t& seed, Tuple const& tuple) {
			HashValueImpl<Tuple, Index - 1>::apply(seed, tuple);
			hash_combine(seed, std::get<Index>(tuple));
		}
	};
	template <class Tuple>
	struct HashValueImpl<Tuple, 0> {
		static void apply(size_t& seed, Tuple const& tuple) {
			hash_combine(seed, std::get<0>(tuple));
		}
	};
	template <typename ... TT>
	struct hash<std::tuple<TT...>> {
		size_t operator()(std::tuple<TT...> const& tt) const {
			size_t seed = 0;
			HashValueImpl<std::tuple<TT...> >::apply(seed, tt);
			return seed;
		}
	};
}
template<class T>int id(vector<T> &a, T b) {
	return lower_bound(ALL(a), b) - a.begin();
}
ll pow(ll base, ll i, ll mod) {
	ll a = 1;
	while (i) {
		if (i & 1) {
			a *= base;
			a %= mod;
		}
		base *= base;
		base %= mod;
		i /= 2;
	}
	return a;
}
ll gcd(ll a, ll b) {
	while (b) {
		ll c = a%b;
		a = b;
		b = c;
	}
	return a;
}
ll lcm(ll a, ll b) {
	return a / gcd(a, b)*b;
}
#ifdef _MSC_VER
#include <intrin.h>
#endif
int popcnt(unsigned long long a) {
#ifndef _MSC_VER
	return __builtin_popcountll(a);
#elif defined _WIN64
	return _mm_popcnt_u64(a);
#else
	return _mm_popcnt_u32(a >> 32) + _mm_popcnt_u32(a);
	a = (a & 0x5555555555555555) + (a >> 1 & 0x5555555555555555);
	a = (a & 0x3333333333333333) + (a >> 2 & 0x3333333333333333);
	a = (a & 0x0f0f0f0f0f0f0f0f) + (a >> 4 & 0x0f0f0f0f0f0f0f0f);
	a = (a & 0x00ff00ff00ff00ff) + (a >> 8 & 0x00ff00ff00ff00ff);
	a = (a & 0x0000ffff0000ffff) + (a >> 16 & 0x0000ffff0000ffff);
	return (a & 0xffffffff) + (a >> 32);
#endif
}
int BitScanF(unsigned long long a) {
#ifndef _MSC_VER
	return __builtin_ctzll(a);
#elif defined _WIN64
	unsigned long ret;
	_BitScanForward64(&ret, a);
	return a;
#else
	unsigned long ret;
	if (!(unsigned long)a){
		_BitScanForward(&ret, a);
		ret += 32;
	}
	else _BitScanForward(&ret, (unsigned long)a);
	return ret;
#endif
}
int BitScanR(unsigned long long a) {
#ifndef _MSC_VER
	return 63 - __builtin_clzll(a);
#elif defined _WIN64
	unsigned long ret;
	_BitScanReverse64(&ret, a);
	return a;
#else
	unsigned long ret;
	if (a >> 32) {
		_BitScanReverse(&ret, a);
		ret += 32;
	}
	else _BitScanReverse(&ret, (unsigned long)a);
	return ret;
#endif
}
template<class T>
class matrix {
public:
	vector<valarray<T>> obj;
	pair<int, int> s;
public:
	matrix(pair<int, int> size, T e = 0) :matrix(size.first, size.second, e) {}
	matrix(int n, int m = -1, T e = 0) :obj(n, valarray<T>(e, m == -1 ? n : m)), s(n, m == -1 ? n : m) {}
	static matrix e(int n) {
		matrix a = (n);
		for (int i = 0; i < n; i++)a[i][i] = 1;
		return a;
	}
	matrix& operator+=(const matrix &p) {
		if (s != p.s)throw runtime_error("matrix error");
		for (int i = 0; i < s.first; i++)
			for (int j = 0; j < s.second; j++)obj[i][j] += p.obj[i][j];
		return *this;
	}
	matrix operator+(const matrix &p) {
		matrix res(*this);
		return res += p;
	}
	matrix& operator-=(const matrix &p) {
		if (s != p.s)throw runtime_error("matrix error");
		for (int i = 0; i < s.first; i++)
			for (int j = 0; j < s.second; j++)obj[i][j] -= p.obj[i][j];
		return *this;
	}
	matrix operator-(const matrix &p) {
		matrix res(*this);
		return res -= p;
	}
	matrix& operator*=(T p) {
		for (auto &a : obj)
			for (auto &b : a)b *= p;
		return *this;
	}
	matrix operator*(T p) {
		matrix res(*this);
		return res *= p;
	}
	matrix operator*(const matrix &p) {
		if (s.second != p.s.first)throw runtime_error("matrix error");
		matrix ret(s.first, p.s.second);
		for (int i = 0; i < s.first; i++)
			for (int j = 0; j < s.second; j++)ret[i] += obj[i][j] * p.obj[j];
		return ret;
	}
	matrix &operator*=(const matrix &p) {
		return *this = *this*p;
	}
	bool operator==(const matrix&p) {
		if (s != p.s)return 0;
		for (int i = 0; i<s.first; i++)for (int j; j< s.second; j++)if (obj[i][j] != p.obj[i][j])return 0;
		return 1;
	}
	pair<int, int> size() const {
		return s;
	}
	matrix &mod(T m) {
		for (auto &a : obj)
			for (auto &b : a)b %= m;
		return *this;
	}
	valarray<T>& operator[](int t) {
		return obj[t];
	}
	void gauss() {
		if (size().first + 1 != size().second)return;
		rep(i, size().first) {
			int p = i;
			repi(j, i, size().first)if (abs(obj[j][i]) > abs(obj[p][i]))p = j;
			swap(obj[i], obj[p]);
			if (abs(obj[i][i]) < 1e-8)return;//contniue;
			repi(j, i + 1, size().second)obj[i][j] /= obj[i][i];
			rep(j, size().first) {
				if (i != j) {
					repi(k, i + 1, size().second)obj[j][k] -= obj[j][i] * obj[i][k];
				}
			}
		}
	}
};
template<class T>
std::pair<matrix<T>, vector<int>> LU_decomposition(matrix<T> a) {
	if(a.size().first!=a.size().second)throw runtime_error("matrix error");
	std::vector<int> pi(a.size().first);
	std::iota(ALL(pi), 0);
	valarray<T> tmp(a.size().first);
	for (int i = 0; i < a.size().first; i++) {
		//int pivot = i;
		//T max = abs(a[i][i]);
		//for (int j = i + 1; j < a.size().first; j++) {
		//	if (max < abs(a[j][i])) {
		//		max = abs(a[j][i]);
		//		pivot = j;
		//	}
		//}
		//std::swap(i, pivot);
		//pi.push_back(pivot);
		std::slice slice(i + 1, a.size().first - i - 1, 1);
		for (int j = i + 1; j < a.size().first; j++) {
			tmp[slice] = a[i][slice];
			tmp *= a[j][i] / a[i][i];
			a[j][slice] -= tmp[slice];
			a[j][i] = a[j][i] / a[i][i];
		}
	}
	return std::make_pair(std::move(a), std::move(pi));
}
template<class T>
matrix<T>LU_solve(pair<matrix<T>, std::vector<int>> a, matrix<T> b) {
	auto pi = std::move(a.second);
	auto A = std::move(a.first);
	if (A.size().first != A.size().second || A.size().first != b.size().first)throw runtime_error("matrix error");
	for (int i = 0; i < A.size().first; i++) {
		std::swap(b[i], b[pi[i]]);
	}
	for (int i = 0; i < A.size().first; i++) {
		for (int j = 0; j < i; j++)b[i] -= A[i][j] * b[j];
	}
	for (int i = A.size().first - 1; i >= 0; i--) {
		for (int j = i + 1; j < A.size().first; j++)b[i] -= A[i][j] * b[j];
		b[i] /= A[i][i];
	}
	return b;
}
template<class T> inline
matrix<T> pow(matrix<T> &base, unsigned long long exp) {
	auto base_(base);
	if (base_.size().first != base_.size().second)throw runtime_error("matrix error");
	matrix<T> res = matrix<T>::e(base_.size().first);
	for (;;) {
		if (exp & 1)res *= base_;
		if (res.obj[0].size() > 100) {
			int a=0;
		}
		if (!(exp /= 2))break;
		base_ *= base_;
	}
	return res;
}
template<class T> inline
matrix<T> modpow(matrix<T> &base, unsigned long long exp, ll m) {
	auto base_(base);
	if (base.size().first != base_.size().second)throw runtime_error("matrix error");
	matrix<T> res = matrix<T>::e(base_.size().first);
	for (;;) {
		if (exp & 1)(res *= base_).mod(m);
		if (res.obj[0].size() > 100) {
			int a = 0;
		}
		if (!(exp /= 2))break;
		(base_ *= base_).mod(m);
	}
	return res;
}
class unionfind {
	vector<int> par, rank, size_;//速度ではなくメモリ効率を考えるならrankのかわりにsizeを使う
public:
	unionfind(int n) :par(n), rank(n), size_(n, 1) {
		iota(ALL(par), 0);
	}
	int find(int x) {
		if (par[x] == x)return x;
		return par[x] = find(par[x]);
	}
	void unite(int x, int y) {
		x = find(x), y = find(y);
		if (x == y)return;
		if (rank[x] < rank[y])swap(x, y);
		par[y] = x;
		size_[x] += size_[y];
		if (rank[x] == rank[y])rank[x]++;
	}
	bool same(int x, int y) {
		return find(x) == find(y);
	}
	int size(int x) {
		return size_[find(x)];
	}
};
//end of lib
//template<class S=void,int ptr_num, class T = char>class trie {
//	umap<T, trie<S, ptr_num, T> next;
//public:
//	S key;
//	trie<S, ptr_num, T>* ptr[ptr_num] = {};
//	trie(S &&data) :key(data) {}
//	trie(const S &data) :key(data) {}
//	void add(T x,S data) {
//		if (!next.find(x))next.insert(x, data);
//	}
//	trie& operator[](T x) {
//		return next[x];
//	}
//	bool find(T x) {
//		retun next.find(x);
//	}
//};
//template<class T=char>class AhoCorasick {
//	trie<pair<bool,int>, 2, T> tree;
//	AhoCorasick(vector<string> p) {
//		int num = 0;
//		vector<decltype(&tree)> que(p.size(),&tree);
//		for (int i = 0;; i++) {
//			bool end = 1;
//			int i = 0;
//			for (auto a : p) {
//				if (i >= a.size())break;
//				end = ;0
//				que[i] = (*que[i])[a[i]];
//				i++;
//			}
//			if (end)break;
//		}
//	}
//};
template<class T, class Func = function<T(T, T)>>
class segtree {
	vector<T> obj;
	int offset;
	Func updater;
	T e;
	int bufsize(int num) {
		int i = 1;
		for (; num >i; i <<= 1);
		offset = i - 1;
		return (i << 1) - 1;
	}
	T query(int a, int b, int k, int l, int r) {
		if (r <= a || b <= l)return e;
		if (a <= l && r <= b)return obj[k];
		else return updater(query(a, b, k * 2 + 1, l, (l + r) / 2), query(a, b, k * 2 + 2, (l + r) / 2, r));
	}
public:
	T query(int a, int b) {//[a,b)
		return query(a, b, 0, 0, offset + 1);
	}
	void updateall(int l = 0, int r = -1) {
		if (r < 0)r = offset + 1;
		l += offset, r += offset;
		if (l == 0)return;
		do {
			l = l - 1 >> 1, r = r - 1 >> 1;
			for (int i = l; i < r; i++)obj[i] = updater(obj[i * 2 + 1], obj[i * 2 + 2]);
		} while (l);
	}
	void update(int k, T &a) {
		k += offset;
		obj[k] = a;
		while (k) {
			k = k - 1 >> 1;
			obj[k] = updater(obj[k * 2 + 1], obj[k * 2 + 2]);
		}
	}
	segtree(int n, T e, const Func &updater = Func()) :obj(bufsize(n), e), e(e), updater(updater) {}
	segtree(vector<T> &vec, T e, const Func &updater = Func()) :obj(bufsize(vec.size()), e), e(e), updater(updater) {
		copy(vec.begin(), vec.end(), obj.begin() + offset);
		updateall();
	}
	T& operator[](int n) {
		return obj[n + offset];
	}
};
template<class T = int>
class BIT {//多次元BITはループをネストすればいいらしい。
	vector<T> bit;
public:
	BIT(int n) :bit(n + 1, 0) {}
	void add(int i, T x) {
		i++;
		while (i <= bit.size()) {
			bit[i - 1] += x;
			i += i&-i;
		}
	}
	T sum(int i) {
		T s = 0;
		i++;
		while (i) {
			s += bit[i - 1];
			i &= i - 1;
		}
		return s;
	}
};
template<class T>
class rangeadd {
	BIT<T> b0, b1;
	int n;
	rangeadd(int n) :b0(n), b1(n), n(n) {}
	void add(int l, int r, T x) {//[l,r)
		b0.add(l, -x(l - 1));
		b1.add(l, x);
		b0.add(r, x*r);
		b1.add(r, -x);
	}
	T sum(int i) {
		return b0.sum(i) + b1.sum(i)*i;
	}
};
class Flow {
	int V;
	struct edge { int to, cap, rev, cost, add; };
	vector<vector<edge>> G;
	vector<int> level, iter, h, dist, prevv, preve;
	void bfs(int s) {
		fill(level.begin(), level.end(), -1);
		queue<int> que;
		level[s] = 0;
		que.push(s);
		while (!que.empty()) {
			int v = que.front(); que.pop();
			for (int i = 0; i < G[v].size(); i++) {
				edge &e = G[v][i];
				if (e.cap > 0 && level[e.to] < 0) {
					level[e.to] = level[v] + 1;
					que.push(e.to);
				}
			}
		}
	}
	int dfs(int v, int t, int f) {
		if (v == t)return f;
		for (int &i = iter[v]; i < G[v].size(); i++) {
			edge &e = G[v][i];
			if (e.cap > 0 && level[v] < level[e.to]) {
				int d = dfs(e.to, t, min(f, e.cap));
				if (d > 0) {
					e.cap -= d;
					G[e.to][e.rev].cap += d;
					return d;
				}
			}
		}
		return 0;
	}
	int res = 0;
	int flow = 0;
	int s, t;
	int max_flow(int s, int t, int fmax = numeric_limits<int>::max()) {
		level.resize(V);
		iter.resize(V);
		for (;;) {
			bfs(s);
			if (level[t] < 0)return flow;
			fill(iter.begin(), iter.end(), 0);
			int f;
			while ((f = dfs(s, t, fmax))>0) {
				flow += f;
				fmax -= f;
			}
			if (fmax == 0)return flow;
		}
	}
	typedef pair<int, int> P;
	int min_cost_flow(int s, int t, int f) {
		h.resize(V);
		dist.resize(V);
		prevv.resize(V);
		preve.resize(V);
		fill(h.begin(), h.end(), 0);
		while (f > 0) {
			priority_queue<P, vector<P>, greater<P>> que;
			fill(dist.begin(), dist.end(), numeric_limits<int>::max());
			dist[s] = 0;
			que.push({ 0,s });
			while (!que.empty()) {
				P p = que.top(); que.pop();
				int v = p.second;
				if (dist[v] < p.first)continue;
				for (int i = 0; i < G[v].size(); i++) {
					edge &e = G[v][i];
					if (e.cap > 0 && dist[e.to]>dist[v] + e.cost + h[v] - h[e.to]) {
						dist[e.to] = dist[v] + e.cost + h[v] - h[e.to];
						prevv[e.to] = v;
						preve[e.to] = i;
						que.push({ dist[e.to],e.to });
					}
				}
			}
			if (dist[t] == numeric_limits<int>::max()) {
				return -1;
			}
			for (int v = 0; v < V; v++)h[v] += dist[v];

			int d = f;
			for (int v = t; v != s; v = prevv[v]) {
				d = min(d, G[prevv[v]][preve[v]].cap);
			}
			f -= d;
			res += d * h[t];
			for (int v = t; v != s; v = prevv[v]) {
				edge &e = G[prevv[v]][preve[v]];
				e.cap -= d;
				G[v][e.rev].cap += d;
			}
		}
		return res;
	}
public:
	Flow(int size, int s, int t) :G(size + 1), V(size + 1), s(s), t(t) {
	}
	void add_edge(int from, int to, int cap, int cost = 0) {
		G[from].push_back(edge{ to, cap, (int)G[to].size(),cost,1 });
		G[to].push_back(edge{ from,0,(int)G[from].size() - 1,-cost,0 });
	}
	void remove_edge_max(int from, int to, int cap, int cost = 0) {
		for (auto &x : G[from]) {
			if (!x.add || x.to != to || x.cap + G[to][x.rev].cap != cap || x.cost != cost)continue;
			int prev = flow;
			int cap = G[to][x.rev].cap;
			G[to][x.rev].cap = 0;
			x.cap = 0;
			cap -= max_flow(from, to, cap) - prev;
			max_flow(t, to, cap);
			max_flow(from, s, cap);
			flow = prev - cap;
			break;
		}
	}
	void remove_edge_cost(int from, int to, int cap, int cost = 0) {
		for (auto &x : G[from]) {
			if (!x.add || x.to != to || x.cap + G[to][x.rev].cap != cap || x.cost != cost)continue;
			cost += G[to][x.rev].cap*G[to][x.rev].cost;
			int cap = G[to][x.rev].cap;
			G[to][x.rev].cap = 0;
			x.cap = 0;
			min_cost_flow(from, to, cap);
			break;
		}
	}
	int max_flow() {
		return max_flow(s, t);
	}
	int min_cost_flow(int f) {
		return min_cost_flow(s, t, f);
	}
};
int main() {
	int n, a, s, g;
	while (cin >> n >> a >> s >> g, n) {
		vector<vector<pair<int,string>>> e(n);
		rep(i, a) {
			int x, y;
			string l;
			cin >> x >> y >> l;
			e[x].emplace_back(y, l);
		}
		vector<vector<string>> dp(500,vector<string>(n));
		for (auto &x : e[s]) {
			if (dp[x.second.length()][x.first].empty() || dp[x.second.length()][x.first] > x.second)dp[x.second.length()][x.first] = x.second;
		}
		rep(i, 480) {
			rep(j, n) {
				if (dp[i][j].empty())continue;
				for (auto &x : e[j]) {
					auto &p = dp[i + x.second.size()][x.first];
					auto buf = dp[i][j] + x.second;
					if (p.empty() || p > buf)p = move(buf);
				}
			}
		}
		string ans, ans2;
		rep(i, 240) {
			if (ans.empty() || ans > dp[i][g] && !dp[i][g].empty())ans = dp[i][g];
		}
		rep(i, 480) {
			if (ans2.empty() || ans2 > dp[i][g] && !dp[i][g].empty())ans2 = dp[i][g];
		}
		if (ans.empty() || ans != ans2) {
			cout << "NO" << endl;
		}
		else cout << ans << endl;
	}

}
//template<class T, class map = std::unordered_map<T,unique_ptr<node>>>
/*class AhoCorasick {
	struct node {
		map<char,unique_ptr<node>> next;
		node* fail = nullptr, *match_list = nullptr;
		std::vector<int> match;
	}root;
	int pattern;
	//template<class string>
	AhoCorasick(std::vector<string> &vs) :pattern(vs.size()) {
		root.fail = &root;
		for (int i = 0; i < vs.size(); i++) {
			node* now = &root;
			for (auto c : vs[i]) {
				if (!now->next[c])now->next[c]=make_unique<node>();
				now = now->next[c].get();
			}
			now->match.push_back(i);
		}
		std::queue<node*> que;
		que.push(&root);
		while (!que.empty()) {
			auto now = que.front();
			que.pop();
			for (auto &next : now->next) {
				if (!next.second)continue;
				if (now->fail->next.count(next.first))next.second->fail = now->fail->next[next.first].get();
				else next.second->fail = now->fail->fail;
				//next.second->match.insert(next.second->match.end(), next.second->fail->match.begin(), next.second->fail->match.end());
				if (next.second->fail->match.empty())next.second->match_list = next.second->fail->match_list;
				else next.second->match_list = next.second->fail;
				que.push(next.second.get());
			}
		}
	}
	auto match_n(string str) {
		vector<int> num(pattern);
		
	}
	auto match_list(string str) {
		vector<pair<int, int>> list;
		auto now = &root;
		for (int i = 0; i < str.size(); i++) {
			if (now->next.count(str[i]))now = now->next[str[i]].get();
			else now = now->fail;
			auto match = now->match_list;
			do {
				match
			}
		}
	}
};*/


Measured memory_bytes: 11944