#include <fstream>
#define Nmax 30001
#include <queue>
using namespace std;

vector <pair <int, int> > L[Nmax];
int d[Nmax], n, m, viz[Nmax], ps, pe;
queue <int> q;

void Citire()
{
    ifstream f("sate.in");
    int x, y, dist;
    f >> n >> m >> ps >> pe;
    for(int i = 1; i <= m; i++)
    {
        f >> x >> y >> dist;
        if(y < x) swap(x, y);
        L[x].push_back(make_pair(y, dist));
        L[y].push_back(make_pair(x, -dist));
    }
    f.close();
}

void Lee(int s)
{
    q.push(s);
    viz[s] = 1;
    d[s] = 0;
    while(!q.empty())
    {
        int k = q.front();
        q.pop();
        for(unsigned int i = 0; i < L[k].size(); i++)
        {
            int j = L[k][i].first;
            int di = L[k][i].second;
            if(viz[j] == 0)
            {
                viz[j] = 1;
                d[j] = d[k] + di;
                q.push(j);
            }
        }
    }
}

int main()
{
    Citire();
    Lee(ps);
    ofstream fout("sate.out");
    fout << d[pe] << "\n";
    return 0;
}