#include <bits/stdc++.h>
using namespace std;

ifstream f("sate.in");
ofstream g("sate.out");

const int NMAX = 30005;
vector<int>dist;
bitset<NMAX>used;
vector<vector<pair<int, int>>> graph;
int n, m, x, y;

void bfs(int node)
{
    queue<int>q;
    q.push(node);

    dist[node]=0;
    used[node]=1;

    while(!q.empty())
    {
        int current =q.front();
        q.pop();

        for(auto coord:graph[current]) {
            int next = coord.first;
            int distance = coord.second;

            if (!used[next]) {
                dist[next]=dist[current] + distance;
                used[next]=1;
                q.push(next);
            }
        }
    }
}

int main() {
    f >> n >> m >> x >> y;
    dist.assign(n+1,0);
    graph.assign(n + 1,vector<pair<int, int>>());
    for(int i=1; i<=m; i++)
    {
        int x, y, d;
        f>>x>>y>>d;
        graph[x].push_back({y, d});
        graph[y].push_back({x, -d});
    }
    bfs (x);
    g << abs(dist[y]) << endl;
    return 0;
}