#include <fstream>
#include <vector>
#include <queue>
#define x first
#define c second
using namespace std;
ifstream f("sate.in");
ofstream g("sate.out");

int N, M, X, Y, a, b, c, D[30005];
vector < pair < int, int> > G[30005];
bool viz[30005];

void bfs()
{
    queue < int > Q;
    Q.push(X); viz[X]=1;

    while (Q.size())
    {
        int aux=Q.front(); Q.pop();
        for (int i=0; i<G[aux].size(); ++i)
            if (!viz[G[aux][i].x])
            {
                viz[G[aux][i].x]=1;
                D[G[aux][i].x]=D[aux]+G[aux][i].c;
                Q.push(G[aux][i].x);
            }
    }
    g<<D[Y]<<'\n';
}

int main()
{
    f>>N>>M>>X>>Y;
    for (int i=1; i<=M; ++i)
    {
        f>>a>>b>>c;
        G[a].push_back(make_pair(b, c));
        G[b].push_back(make_pair(a, -c));
    }

    bfs();
    return 0;
}