#include <fstream>
#include <vector>
#include <queue>
#include <stdlib.h>
using namespace std;
ifstream fin("sate.in");
ofstream fout("sate.out");
const int NMAX=30000;
struct graf
{
    int nod, distanta;
};
vector <graf>G[NMAX+5];
queue <int>q;
bool viz[NMAX+5];
int s[NMAX+5];
int x, y;
void bfs(int nod)
{
    int i;
    viz[nod]=1;
    s[nod]=0;
    q.push(nod);
    while(!q.empty())
    {
        nod=q.front();
        q.pop();
        for(i=0;i<G[nod].size();i++)
        {
             if(viz[G[nod][i].nod]==0)
             {
               s[G[nod][i].nod]=G[nod][i].distanta+s[nod];
               q.push(G[nod][i].nod);
               viz[G[nod][i].nod]=1;
             }
        }
    }
}
int main()
{
    int n, m, a, b, d, i;
    graf aux;
    fin>>n>>m>>x>>y;
    for(i=1;i<=m;i++)
    {
        fin>>a>>b>>d;
        aux.distanta=d;
        aux.nod=b;
        G[a].push_back(aux);
        aux.distanta=-d;
        aux.nod=a;
        G[b].push_back(aux);
    }
    bfs(x);
    fout<<s[y]<<"\n";



    return 0;
}