#include <iostream>
#include <fstream>
#include <vector>
#include <queue>
#define NMax 30010
#define MMax 100035

using namespace std;

int n, m, source, destination;
int dist[NMax];
bool viz[NMax];
struct graph
{
    int node, distance;
    graph ()
    {
        node = distance = 0;
    }
    graph (const int _node, const int _distance)
    {
        node = _node;
        distance = _distance;
    }
};
vector <graph> G[NMax];

void Read()
{
    ifstream f("sate.in");
    f>>n>>m>>source>>destination;
    int i, x, y, cost;
    for (i=1; i<=m; ++i)
    {
        f>>x>>y>>cost;
        G[x].push_back(graph(y, cost));
        G[y].push_back(graph(x, cost));
    }
    f.close();
}

void Solve()
{
    queue <int> q;
    q.push(source);
    viz[source] = true;
    dist[source] = 0;

    int node;

    while (!viz[destination])
    {
        node = q.front();
        q.pop();
        graph aux;
        for (vector <graph>::iterator it = G[node].begin(); it!=G[node].end(); ++it)
        {
            aux = *it;
            if (!viz[aux.node])
            {
                viz[aux.node] = true;
                if (aux.node < node)
                    dist[aux.node] = dist[node] - aux.distance;
                else
                    dist[aux.node] = dist[node] + aux.distance;
                q.push(aux.node);
            }
        }
    }
}

void Write()
{
    ofstream g("sate.out");
    g<<dist[destination]<<"\n";
    g.close();
}

int main()
{
    Read();
    Solve();
    Write();
    return 0;
}