#include <fstream>
#include <vector>
#include <queue>
using namespace std;
ifstream f("sate.in");
ofstream g("sate.out");
int n,m,x,y,x1,y1,d,used[30001],dist[30001];
vector < pair<int,int> > graf[30001];
queue <int> q;
void bfs()
{
    while(!q.empty())
    {
        int k=q.front();
        q.pop();
        for(int it=0;it<graf[k].size();it++)
        {
            if(!used[graf[k][it].first])
            {
                used[graf[k][it].first]=1;
                dist[graf[k][it].first]=dist[k]+graf[k][it].second;
                q.push(graf[k][it].first);
            }
        }
    }
}
int main()
{
    f>>n>>m>>x>>y;
    for(int i=1;i<=m;i++)
    {
        f>>x1>>y1>>d;
        graf[x1].push_back(make_pair(y1,d));
        graf[y1].push_back(make_pair(x1,-d));
    }
    used[x]=1;
    q.push(x);
    bfs();
    g<<dist[y];
    return 0;
}