#include    <iostream>
#include    <fstream>
#include    <vector>
#include    <queue>
using namespace std;
ifstream fin("sate.in");
ofstream fout("sate.out");
const int NLIM = 100005;
int N, M, x,y,d,i,j;
int Distance[NLIM],viz[NLIM];
vector < pair <int,int> > Edge[NLIM];
queue <int> Q;

void BFS(int s)
{
    int Node, Next,k;
    Q.push(s);
    viz[s]=1;
    while(!Q.empty())
    {
        Node =  Q.front();
        Q.pop();
        for(unsigned int k = 0; k < Edge[Node].size(); k++)
        {

            if(viz[Edge[Node][k].first]==0)
            {   Q.push(Edge[Node][k].first);
                viz[Edge[Node][k].first]=1;
                if(Node>Edge[Node][k].first) Distance[Edge[Node][k].first]=Distance[Node]-Edge[Node][k].second;
                   else Distance[Edge[Node][k].first]=Distance[Node]+Edge[Node][k].second;
            }
        }
    }
}
void Read()
{   int k;
    fin >> N >> M >> x>>y;
    for(int k = 1; k <= M; k++)
    {

        fin >> i >> j>>d;
        Edge[i].push_back(make_pair(j,d));
        Edge[j].push_back(make_pair(i,d));
    }


    BFS(x);
    fout<<Distance[y];
}
int main()
{
    Read();
    return 0;
}