#include <fstream>
#include <queue>
#include <vector>
 
using namespace std;
 
ifstream fin("sate.in");
ofstream fout("sate.out");
 
int n, m, x, y;
vector< vector < pair<int,int> > >G;
vector<int> d;
vector<bool>viz;
queue<int>Q;
 
int a, b, c;
 
int main()
{
    fin >> n >> m >> x >> y;
 
    G.resize(n + 1);
    d.resize(n + 1);
    viz.resize(n + 1);
 
    for ( int i = 0; i < m; ++i )
    {
        fin >> a >> b >> c;
        G[a].push_back({b,c});
        G[b].push_back({a,-c});
    }
 
    Q.push(x);
    viz[x] = true;
    d[x] = 0;
 
    while ( !Q.empty() )
    {
        int top = Q.front();
        Q.pop();
 
        for ( auto p : G[top] )
        {
            if ( !viz[p.first] ) {
                viz[p.first] = true;
                d[p.first] = d[top] + p.second;
                Q.push(p.first);
		if ( p.first == y ) 
			break;
            }
        }
    }
 
    fout << d[y];
 
    fin.close();
    fout.close();
    return 0;
}