CF 46 Q4

#include <bits/stdc++.h>

using namespace std;

void dbg_out() { cerr << endl; }
template<typename Head, typename... Tail>
void dbg_out(Head H, Tail... T) { cerr << ' ' << H; dbg_out(T...); }
#define dbg(...) cerr << "(" << #__VA_ARGS__ << "):", dbg_out(__VA_ARGS__)

#define rng_init mt19937 rng(chrono::steady_clock::now().time_since_epoch().count())
#define rng_seed(x) mt19937 rng(x)
#define all(x) (x).begin(), (x).end()
#define sz(x) (int) (x).size()
#define int long long

const int MXN = 1e5 + 5, INF = 1e18;

void solve() {
    int M, N, S, H;
    cin >> M >> N >> S >> H;
    S--, H--;

    vector<pair<int, int>> g[MXN];

    for (int i = 0; i < N; i++) {
        int u, v, w;
        cin >> u >> v >> w;
        u--, v--;

        g[u].emplace_back(v, w);
        g[v].emplace_back(u, w);
    }

    priority_queue<pair<int, int>, vector<pair<int, int>>, greater<>> pq;
    vector<int> dist(M + 1, INF);

    dist[S] = 0;
    pq.emplace(0, S);

    while (!pq.empty()) {
        auto [cur_dist, u] = pq.top();
        pq.pop();

        if (cur_dist != dist[u]) continue;

        for (const auto &[v, wt] : g[u]) {
            int new_dist = cur_dist + wt;

            if (new_dist < dist[v]) {
                dist[v] = new_dist;
                pq.emplace(new_dist, v);
            }
        }
    }

    cout << dist[H];
}

signed main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);

    int TC = 1;
    // cin >> TC;
    while (TC--) solve();
}