#include <iostream>
#include <fstream>
#include <vector>
#include <queue>

using namespace std;

ifstream fin("sate.in");
ofstream fout("sate.out");

vector < pair <int,int> > v[30001];
queue <short> q;
int d[30001],n,m,i,x,y,a,b,cost;


int main()
{
    fin>>n>>m>>x>>y;
    for(i=1;i<=m;i++)
    {
        fin>>a>>b>>cost;
        v[a].push_back(make_pair(cost,b));
        v[b].push_back(make_pair(-cost,a));
    }
    q.push(x);
    for(i=1;i<=n;i++)d[i]=-1;
    d[x]=0;
    while(d[y]==-1&&q.empty()==0)
    {
        a=q.front();
        q.pop();
        for(i=0;i<v[a].size();i++)
            if(d[v[a][i].second]==-1)
            {
                d[v[a][i].second]=d[a]+v[a][i].first;
                q.push(v[a][i].second);
            }
    }
    fout<<d[y];
    return 0;
}