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

using namespace std;

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

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

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

int main()
{
    fin>>n>>m>>x>>y;
    for(int i=1;i<=m;i++)
    {
        fin>>a>>b>>c;
        mat[a].push_back({b,c});
        mat[b].push_back({a,-c});
    }
    fout<<parc(x);
    return 0;
}