#include <bits/stdc++.h>
#define maxn 2505
using namespace std;
int a[maxn][maxn];
int n, m, s, t;
int in_queue[maxn];
queue<int> q;
int relax(int u, int v)
{
    if (a[s][u] + a[u][v] < a[s][v])
    {
        a[s][v] = (a[s][u] + a[u][v]);
        return 1;
    }
    return 0;
}
int main()
{
    scanf("%d%d%d%d", &n, &m, &s, &t);
    for (int i(1); i <= n; ++i)
        for (int j(1); j <= n; ++j)
            a[i][j] = 114514;
    while (m--)
    {
        int u, v, w;
        scanf("%d%d%d", &u, &v, &w);
        a[u][v] = w;
        a[v][u] = w;
    }
    q.push(s);
    in_queue[s] = true;
    while (!q.empty())
    {
        int u = q.front();
        q.pop();
        in_queue[u] = false;
        for (int i(1); i <= n; ++i)
            if (relax(u, i) && !in_queue[i])
            {
                q.push(i);
                in_queue[i] = true;
            }
    }
    printf("%d\n", a[s][t]);
    return 0;
}

// SPFA 70 pts ???????

Floyd $O(n ^ 3)$ 能跑 $80pts$ 但是为什么 SPFA 最坏情况 $O(nm)$ 只能跑到 $70pts$ /kk

$1 \leq n \leq 2500$ $1 \leq m \leq 6200$