#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <climits>
#include <cfloat>
#include <ctime>
#include <cassert>
#include <map>
#include <utility>
#include <set>
#include <iostream>
#include <memory>
#include <string>
#include <vector>
#include <algorithm>
#include <functional>
#include <sstream>
#include <complex>
#include <stack>
#include <queue>
#include <numeric>
#include <list>
#include <iomanip>
 
using namespace std;
 
#if __GNUC__
#include <tr1/unordered_map>
#include <tr1/unordered_set>
using namespace tr1;
#else
#include <unordered_map>
#include <unordered_set>
#endif
 
 
 
#ifdef _MSC_VER
#define __typeof__ decltype
template <class T> int popcount(T n) { return n ? 1 + popcount(n & (n - 1)) : 0; }
#endif
#ifdef __GNUC__
template <class T> int popcount(T n);
template <> int popcount(unsigned int n) { return __builtin_popcount(n); }
template <> int popcount(int n) { return __builtin_popcount(n); }
template <> int popcount(unsigned long long n) { return __builtin_popcountll(n); }
template <> int popcount(long long n) { return __builtin_popcountll(n); }
#endif
 
#define rep (i, n) for (int i = 0; i < (int)n; ++i)
#define foreach(it, c) for (__typeof__((c).begin()) it=(c).begin(); it != (c).end(); ++it)
#define rforeach(it, c) for (__typeof__((c).rbegin()) it=(c).rbegin(); it != (c).rend(); ++it)
#define all(c) (c).begin(), (c).end()
#define rall(c) (c).rbegin(), (c).rend()
#define CL(arr, val) memset(arr, val, sizeof(arr))
#define MEMCPY(dest, src) memcpy(dest, src, sizeof(src))
 
 
template <class T> void max_swap(T& a, const T& b) { a = max(a, b); }
template <class T> void min_swap(T& a, const T& b) { a = min(a, b); }
 
typedef long long ll;
typedef pair<int, int> pint;
 
template <class T> string to_s(const T& a) { ostringstream os; os << a; return os.str(); }
 
const double EPS = 1e-8;
const double PI = acos(-1.0);
const int dx[] = { 0, 1, 0, -1 };
const int dy[] = { 1, 0, -1, 0 };
bool valid_pos(int x, int y, int w, int h) { return 0 <= x && x < w && 0 <= y && y < h; }
 
template <class T> void print(T a, int n, int br = 1, const string& deli = ", ") { cout << "{ "; for (int i = 0; i < n; ++i) { cout << a[i]; if (i + 1 != n) cout << deli; } cout << " }"; while (br--) cout << endl; }
template <class T> void print(const vector<T>& v, int br = 1, const string& deli = ", ") { print(v, v.size(), br, deli); }
template <class T> void print2d(T a, int w, int h, int width = -1, int br = 1) { for (int i = 0; i < h; ++i) { for (int j = 0; j < w; ++j) {	if (width != -1) cout.width(width); cout << a[i][j] << ' ';	} cout << endl; } while (br--) cout << endl; }
 
template <class T> void input(int n, T& a) { for (int i = 0; i < n; ++i) cin >> a[i]; }
template <class T, class U> void input(int n, T& a, U& b) { for (int i = 0; i < n; ++i) cin >> a[i] >> b[i]; }
template <class T, class U, class V> void input(int n, T& a, U& b, V& c) { for (int i = 0; i < n; ++i) cin >> a[i] >> b[i] >> c[i]; }
template <class T> vector<T> input(int n) { vector<T> res(n); for (int i = 0; i < n; ++i) cin >> res[i]; return res; }


int n, a, s, g;
string solve()
{
    typedef pair<string, int> P;
    vector<P> e[40];
    for (int i = 0; i < a; ++i)
    {
        int x, y;
        string t;
        cin >> x >> y >> t;
        e[x].push_back(P(t, y));
    }

    const string INF = string(1, 'z' + 1);
    int max_len = n * 6 * 2;
    string dis[42][40 * 6 * 2 + 6];
    for (int i = 0; i < n; ++i)
        for (int j = 0; j <= max_len; ++j)
            dis[i][j] = INF;


    priority_queue<P, vector<P>, greater<P> > q;
    q.push(P("", s));
    dis[s][0] = "";
    while (!q.empty())
    {
        string t = q.top().first;
        int p = q.top().second;
        q.pop();
        if (t > dis[p][t.size()])
            continue;

        foreach (it, e[p])
        {
            int to = it->second;
            string nt = t + it->first;
            if (nt.size() <= max_len && nt < dis[to][nt.size()])
            {
                dis[to][nt.size()] = nt;
                q.push(P(nt, to));
            }
        }
    }

    string res = INF;
    for (int i = 0; i <= max_len; ++i)
        min_swap(res, dis[g][i]);

    if (res == INF || res.size() > 6 * (n - 1))
        return "NO";
    else
        return res;
}
int main()
{
    ios::sync_with_stdio(false);


    while (cin >> n >> a >> s >> g, n)
        cout << solve() << endl;
}

Measured memory_bytes: 8144