#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef pair<ll, ll> P;

#define each(i,a) for (auto&& i : a)
#define FOR(i,a,b) for (ll i=(a),__last_##i=(b);i<__last_##i;i++)
#define RFOR(i,a,b) for (ll i=(b)-1,__last_##i=(a);i>=__last_##i;i--)
#define REP(i,n) FOR(i,0,n)
#define RREP(i,n) RFOR(i,0,n)
#define __GET_MACRO3(_1, _2, _3, NAME, ...) NAME
#define rep(...) __GET_MACRO3(__VA_ARGS__, FOR, REP)(__VA_ARGS__)
#define rrep(...) __GET_MACRO3(__VA_ARGS__, RFOR, RREP)(__VA_ARGS__)
#define pb push_back
#define all(a) (a).begin(),(a).end()
#define chmin(x,v) x = min(x, v)
#define chmax(x,v) x = max(x, v)

const ll linf = 1e18;
const int inf = 1e9;
const double eps = 1e-12;
const double pi = acos(-1);

template<typename T>
istream& operator>>(istream& is, vector<T>& vec) {
    each(x,vec) is >> x;
    return is;
}
template<typename T>
ostream& operator<<(ostream& os, const vector<T>& vec) {
    rep(i,vec.size()) {
        if (i) os << " ";
        os << vec[i];
    }
    return os;
}
template<typename T>
ostream& operator<<(ostream& os, const vector< vector<T> >& vec) {
    rep(i,vec.size()) {
        if (i) os << endl;
        os << vec[i];
    }
    return os;
}

struct Edge {
    ll from, to;
    string s;
};
ll L = 250;
int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    ll n, a, s, g;
    while (cin >> n >> a >> s >> g, n || a || s || g) {
        string infstr = ""; infstr += 'z'+1;
        vector<Edge> edges;
        rep(i, a) {
            ll x, y; string lab; cin >> x >> y >> lab;
            edges.pb({x, y, lab});
        }
        vector<string> dist(n, infstr);
        dist[g] = "";
        auto update = [&]() {
            ll res = 0;
            each(e, edges) {
                if (dist[e.to] == infstr) continue;
                if (e.s + dist[e.to] < dist[e.from]) {
                    dist[e.from] = e.s + dist[e.to];
                    chmax(res, 1LL);
                    if (e.from == s) chmax(res, 2LL);
                }
            }
            return res;
        };
        try {
            rep(t, L) {
                if ( update() == 0 ) break;
            }
            rep(t, n) {
                if ( update() == 2 ) throw 1;
            }
            if (dist[s] == infstr) throw 1;
            cout << dist[s] << endl;
        }
        catch (int err) {
            cout << "NO" << endl;
        }
    }
}