#include <fstream>
#include <vector>
#include <queue>
#include <algorithm>
#include <cstdlib>
using namespace std;

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

vector< pair<int, int> > a[30001];
int d[30001], viz[30001];
int n, m, x, y;

int main()
{
    int i, j, c, k;
    f >> n >> m >> x >> y;
    for (k = 1; k <= m; k++)
    {
        f >> i >> j >> c;
        if (i > j)
            swap(i, j);
        a[i].push_back(make_pair(j, c));
        a[j].push_back(make_pair(i, -c));
    }
    queue<int> q;
    q.push(x);
    viz[x] = 1;
    while (!q.empty())
    {
        int nod = q.front();
        q.pop();
        for (k = 0; k < a[nod].size(); k++)
        {
            int vec = a[nod][k].first;
            int cost = a[nod][k].second;
            if (viz[vec] == 0)
            {
                viz[vec] = 1;
                d[vec] = d[nod] + cost;
                q.push(vec);
            }
        }
    }
    g << abs(d[y]) << "\n";
    return 0;
}