#include <iostream>
#include <vector>
#include <queue>
#include <fstream>
 
using namespace std;
 
int vizitat[100000], suma[100000];
 
int main()
{
    ifstream f("sate.in");
    ofstream g("sate.out");
 
    vector<int> v[100000], dist[100000];
    queue<int> q;
 
    int n, m, i, start, sfarsit, x, y, nod, sursa, col = 1, d;
 
    f >> n >> m >> start >> sfarsit;
 
    for (i = 0; i < m; i++)
        {
            f >> x >> y >> d;
 
            v[x].push_back(y);
            dist[x].push_back(d);
 
            v[y].push_back(x);
            dist[y].push_back(d);
        }
 
    sursa = start;
 
    vizitat[sursa] = col;
    q.push(sursa);
 
    while (!q.empty())
        {
         nod = q.front();
         q.pop();
 
         col = vizitat[nod];
 
         if(col == 1)
            col = 2;
         else
            col = 1;
 
         for(i = 0; i < v[nod].size(); i++)
            if(vizitat[v[nod][i]] == 0)
                {
                    vizitat[v[nod][i]] = col;
 
                    if(nod > v[nod][i])
                        suma[v[nod][i]] = suma[nod] - dist[nod][i];
 
                    if(nod < v[nod][i])
                        suma[v[nod][i]] = suma[nod] + dist[nod][i];
		if(v[nod][i] == sfarsit)
		{g << suma[sfarsit];
		return 0;
		}
 
                    q.push(v[nod][i]);
                }
        }
 
    g << suma[sfarsit];
 
    return 0;
}