#include <iostream>
#include <fstream>
#include <queue>
#include <vector>
#define mp make_pair
using namespace std;

int drum[30005],y;
vector<pair<int,int> > list[30005];
queue<int> q;

void bfs()
{
    while(!q.empty())
    {
        int nod=q.front();
        q.pop();
        if(nod==y)
           return;
        for(unsigned int i=0;i<list[nod].size();i++)
        {
            int next_nod=list[nod][i].first;
            int cost=list[nod][i].second;
            if(!drum[next_nod])
            {
                drum[next_nod]=drum[nod]+cost;
                q.push(next_nod);
            }
        }
    }
    return;
}

int main()
{
    int n,m,x;
    ifstream f("sate.in");
    ofstream g("sate.out");
    f>>n>>m>>x>>y;
    for(int i=1;i<=m;i++)
    {
        int a,b,c;
        f>>a>>b>>c;
        list[a].push_back(mp(b,c));
        list[b].push_back(mp(a,-c));
    }
    drum[x]=0;
    q.push(x);
    bfs();
    g<<drum[y];
    return 0;
}