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

#define maxN 30003

using namespace std;

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

int n,m,x,y,d[maxN];
vector < pair <int,int> > v[maxN];
bitset <maxN> inQ;
queue <int> Q;

void readData()
{
    fin>>n>>m>>x>>y;
    while (m--)
    {
        int a,b,c;
        fin>>a>>b>>c;
        v[a].push_back(make_pair(b,c));
        v[b].push_back(make_pair(a,-c));
    }
}

void BFS(int x)
{
    Q.push(x);
    inQ[x]=1;
    d[x]=0;
    while (!Q.empty())
    {
        x=Q.front();
        Q.pop();
        for (int i=0; i<v[x].size(); ++i)
            if (!inQ[v[x][i].first])
        {
            d[v[x][i].first]=d[x]+v[x][i].second;
            Q.push(v[x][i].first);
            inQ[v[x][i].first]=1;
        }
    }
    fout<<d[y];
}

int main()
{
    readData();
    BFS(x);
    return 0;
}