#include <bits/stdc++.h>
#define Nmax 30005
#define inf 1<<30

using namespace std;

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

vector < pair<int,int> > mat[Nmax];
int n,m,x,y,a,b,c,d[Nmax],viz[Nmax];
queue <int> coada;

void bfs(int vf)
{
    coada.push(vf);
    d[vf]=0;
    while(!coada.empty())
    {
        x=coada.front();
        coada.pop();
        for(int i=0;i<mat[x].size();i++)
        {
            if(!viz[mat[x][i].first])
            {
                viz[mat[x][i].first]=1;
                coada.push(mat[x][i].first);
                d[mat[x][i].first]=d[x]+mat[x][i].second;
                if(mat[x][i].first==y)
                    exit;
            }
        }
    }

}

int main()
{
    fin>>n>>m>>x>>y;
    for(;m--;)
    {
        fin>>a>>b>>c;
        mat[a].push_back({b,c});
        mat[b].push_back({a,-c});
    }
    bfs(x);
    fout<<d[y];
    return 0;
}