#include <iostream>
#include <fstream>
#include <vector>
#include <queue>
#define MAX_N 30005
using namespace std;

ifstream f("sate.in");
ofstream g("sate.out");

vector <int> G[MAX_N], D[MAX_N];
int dist[MAX_N], X, Y;

void BFS(int nod)
{
    queue <int> Q;
    Q.push(nod);
    while(!Q.empty())
    {
        int n = Q.front();
        for(int i = 0; i < G[n].size(); i++)
        {
            if(dist[G[n][i]] == 0)
            {
                if(G[n][i] > n)
                    dist[G[n][i]] = dist[n] + D[n][i];
                else
                    dist[G[n][i]] = dist[n] - D[n][i];
                Q.push(G[n][i]);
                if(G[n][i] == Y)
                    return;
            }
        }
        Q.pop();
    }
}


int main()
{
    int N, M, a, b, c, i;
    f >> N >> M >> X >> Y;
    for(i = 1; i <= M; i++)
    {
        f >> a >> b >> c;
        G[a].push_back(b);
        G[b].push_back(a);
        D[a].push_back(c);
        D[b].push_back(c);
    }
    BFS(X);
    g << dist[Y];
    return 0;
}