#include <fstream>
#include <queue>
#include <vector>
using namespace std;
ifstream fin("sate.in");
ofstream fout("sate.out");
vector<pair<int,int> > H[30001];
queue<int>Q;
bool viz[30001];
int dist[30001];
void bfs(int start,int y)
{
    viz[start]=1;
    Q.push(start);
    while(Q.size())
    {
        if(dist[y]>0) return;
        int a=Q.front();
        Q.pop();
        for(vector<pair<int,int> > ::iterator it=H[a].begin();it!=H[a].end();it++)
        {
            if(viz[it->first]==0)
            {
                dist[it->first]=dist[a]+it->second;
                Q.push(it->first);
                viz[it->first]=1;
            }
        }
    }
}
int main()
{
    int n,m,i,a,b,c,x,y;
    fin>>n>>m>>x>>y;
    for(i=1;i<=m;i++)
    {
        fin>>a>>b>>c;
        H[a].push_back(make_pair(b,c));
        H[b].push_back(make_pair(a,-c));
    }
    bfs(x,y);
    fout<<dist[y];
}