#include <fstream>
#include <vector>
#include <queue>
#include <stdlib.h>
#include <cstdio>
#include <bitset>
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;
bitset <NMAX+5> viz;
int s[NMAX+5];
int x, y;
void bfs(int nod)
{
    int i;
    int a;
    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++)
        {
            a=G[nod][i].nod;
             if(viz[a]==0)
             {
               s[a]=G[nod][i].distanta+s[nod];
               q.push(a);
               viz[a]=1;
             }
        }
        if(s[y]!=0)
        {
              printf("%d", s[y]);
              exit(0);
        }
    }
}
int main()
{
    int n, m, a, b, d, i;
    graf aux;
    freopen("sate.in", "r", stdin);
    freopen("sate.out", "w", stdout);
    scanf("%d %d %d %d", &n, &m, &x, &y);
    for(i=1;i<=m;i++)
    {
        scanf("%d %d %d", &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);



    return 0;
}