#include <fstream>
#include <vector>
using namespace std;
vector <int> L[30010];
vector <int> D[30010];
ifstream fin ("sate.in");
ofstream fout ("sate.out");
int n, m, i, j, dist[30010], x, y, f[30010], ii, jj, d;
void dfs (int nod)
{
    f[nod]=1;
    for (int i=0; i<L[nod].size(); i++)
    {
        int vecin=L[nod][i];
        if (f[vecin]==0)
        {
            dist[vecin]=dist[nod]+D[nod][i];
            dfs(vecin);
        }
    }
}
int main()
{
    fin>>n>>m>>x>>y;
    for (i=1; i<=m; i++)
    {
        fin>>ii>>jj>>d;
        L[ii].push_back(jj);
        D[ii].push_back(d);
        L[jj].push_back(ii);
        D[jj].push_back(-d);
    }
    dfs(x);
    fout<<dist[y];
}