#include <iostream>
#include <fstream>
#include <queue>
#include <vector>

using namespace std;

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

int n, m, a, b, x, y, d, dist[30002], viz[30002];
vector<pair<int,int> > liste[30002];

int main()
{
    f >> n >> m >> a >> b;

    for( int i=1; i<=m; i++ )
    {
        f >> x >> y >> d;
        liste[x].push_back(make_pair(y,d));
        liste[y].push_back(make_pair(x,-d));
    }

    queue<int>q;
    viz[a] = 1;
    dist[a] = 0;
    q.push(a);

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

        for( vector<pair<int,int> >::iterator it=liste[x].begin(); it!=liste[x].end(); it++ )
            if( viz[it->first] == 0 )
            {
                viz[it->first] = 1;
                dist[it->first] = dist[x] + it->second;
                q.push(it->first);
            }
    }

    g << dist[b];

    return 0;
}