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

const int NMAX = 30000;

int n, m, x, y;
vector<pair<int, int>> graf[NMAX + 1];
int d[NMAX + 1];

void BFS(int nod)
{
    for (int i = 1; i <= n; i ++)
    {
        d[i] = NMAX + 1;
    }
    d[nod] = 0;
    queue<int> q;
    q.push(nod);
    while (!q.empty())
    {
        int a = q.front();
        if (a == y)
            break;
        q.pop();
        for (auto [i, j] : graf[a])
        {
            d[i] = d[a] + j;
            q.push(i);
        }
    }
}

int main() 
{
    ifstream cin("sate.in");
    ofstream cout("sate.out");
	cin >> n >> m >> x >> y;
    for (int i = 1; i <= m; i ++)
    {
        int a, b, c; cin >> a >> b >> c;
        graf[a].push_back({b, c});
        graf[b].push_back({a, -c});
    }
    BFS(x);
    cout << abs(d[y]);
}