#include <fstream>
#include <queue>
#include <vector>
using namespace std;
#define NMAX 30005
ifstream fin("sate.in");
ofstream fout("sate.out");
int x1,y1,n,m,d[NMAX];
bool viz[NMAX];
queue <int> qnod,qval;
vector <int> v[NMAX],cost[NMAX];
void read()
{
    fin>>n>>m>>x1>>y1;
    int a,b,dist;
    for(int i=1;i<=m;i++)
    {
        fin>>a>>b>>dist;
        v[a].push_back(b);
        v[b].push_back(a);
        cost[a].push_back(dist);
        cost[b].push_back(dist);
    }
}
void tipar()
{
    for(int i=1;i<=n;i++)
    {
        for(int j=0;j<v[i].size();j++)
            fout<<i<<" "<<v[i][j]<<" "<<cost[i][j]<<endl;
    }
}
void bfs()
{
    qnod.push(x1);
    qval.push(0);
    viz[x1]=true;
    int nod,drum,i;
    while(!qnod.empty())
    {
        nod=qnod.front();
        drum=qval.front();
        qnod.pop();
        qval.pop();
        for(i=0;i<v[nod].size();i++)
        {
            if(viz[v[nod][i]])
                continue;
            viz[v[nod][i]]=true;
            if(v[nod][i]>nod)
                d[v[nod][i]] = drum + cost[nod][i];
            else
                d[v[nod][i]] = drum - cost[nod][i];
            qnod.push(v[nod][i]);
            qval.push(d[v[nod][i]]);
        }
    }
}
int main()
{
    read();
    //tipar();
    bfs();
    fout<<d[y1];
}