awd

#include <bits/stdc++.h>

using namespace std;

using ll = long long;

const ll INF = 1e15 + 7;

vector<ll> used;
vector<ll> topsort;

void dfs_ts(ll v, vector<map<ll, ll>> &gr) {
    used[v] = true;
    for (auto [u, k] : gr[v]) {
        if (!used[u]) {
            dfs_ts(u, gr);
        }
    }
    topsort.push_back(v);
}

int main() {
    ll n, m, s, t;
    cin >> n >> m >> s >> t;
    --s;
    --t;
    vector<map<ll, ll>> gr(n);
    for (ll i = 0; i < m; i++) {
        ll a, b, c;
        cin >> a >> b >> c;
        --a;
        --b;
        gr[a][b] = c;
    }
    used.assign(n, false);
    for (ll i = 0; i < n; i++) {
        if (!used[i]) dfs_ts(i, gr);
    }
    reverse(topsort.begin(), topsort.end());
    ll i;
    for (i = 0; i < n; i++) {
        if (topsort[i] == s) {
            break;
        }
    } ll S = i;
    vector<ll> dp(n, INF);
    used.assign(n, false);
    dp[s] = 0;
    for (i = S; i < n; i++) {
        if (dp[topsort[i]] != INF) {
            for (auto [u, k] : gr[topsort[i]]) {
                dp[u] = min(dp[u], dp[topsort[i]] + k);
            }
        }
    }
    if (dp[t] == INF) {
        cout << "Unreachable";
    } else {
        cout << dp[t];
    }
    return 0;
}