#include <bits/stdc++.h>
 
using namespace std;
 
ifstream fin("sate.in");
ofstream fout("sate.out");
 
int i,n,m,x,y,a,b,c,s,nr,mn,sol[50005];
vector < pair <int,int> > t[50005];
queue <int> coada;
 
void bfs(int x)
{
    int i,k;
    coada.push(x);
    while (!coada.empty())
    {
        k=coada.front();
        coada.pop();
        for (i=0; i<t[k].size(); i++)
        {
            if (t[k][i].second+sol[k]<sol[t[k][i].first] || sol[t[k][i].first]==0)
            {
                sol[t[k][i].first]=t[k][i].second+sol[k];
                coada.push(t[k][i].first);
            }
        }
    }
}
 
int main()
{
    fin>>n>>m>>x>>y;
    for (i=1; i<=m; i++)
    {
        fin>>a>>b>>c;
        t[a].push_back(make_pair(b,c));
		t[b].push_back(make_pair(a,-c));
    }
 
    bfs(x);
 
	fout<<sol[y];    

    return 0;
}