#include <bits/stdc++.h>
#define pb push_back
#define mp make_pair
using namespace std;
ifstream fin("sate.in");
ofstream fout("sate.out");
vector <pair<int,int> > v[30001];
int n,m,x,y,a,b,s,d[30001];
void bfs(int x)
{
    queue <int> q;
    q.push(x);
    while(!q.empty())
    {
        int nod=q.front();
        q.pop();
        for(int i=0;i<v[nod].size();i++)
        {
            if(!d[v[nod][i].first]&&v[nod][i].first!=x)
            {
                int next=v[nod][i].first,dist=v[nod][i].second;
                d[next]=d[nod]+dist;
                q.push(next);
                if(next==y)
                {
                    return;
                }
            }
        }
    }
}
int main()
{
    fin>>n>>m>>x>>y;
    for(int i=1;i<=m;i++)
    {
        fin>>a>>b>>s;
        v[a].pb(mp(b,s));
        v[b].pb(mp(a,-s));
    }
    bfs(x);
    fout<<d[y];
    return 0;
}