#include <fstream>
#include <vector>
#define VAL 30005

using namespace std;

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

int N, M, i, j, x, y;
int a, b, c;
int cost[VAL];
bool parc[VAL];
vector< pair<int, int> > G[VAL];

void dfs(int nod)
{
    vector< pair<int, int> > :: iterator it;
    parc[nod]=true;
    for (it=G[nod].begin(); it!=G[nod].end(); it++)
    {
        if (parc[it->first]==false)
        {
            cost[it->first]=cost[nod]+(it->second);
            dfs(it->first);
        }
    }
}

int main()
{
    fin >> N >> M >> x >> y;
    for (i=1; i<=M; i++)
    {
        fin >> a >> b >> c;
        G[a].push_back(make_pair(b, c));
        G[b].push_back(make_pair(a, -c));
    }
    dfs(x);
    fout << cost[y] << '\n';
    fin.close();
    fout.close();
    return 0;
}