#include <iostream>
#include <fstream>
#include <vector>

using namespace std;

ifstream fin  ("sate.in");
ofstream fout ("sate.out");

vector < pair<int, int> > v[30005];
int n, m, x, y, ii, jj, dd;
int st, dr, vcrt, dcrt, nod;
int f[30005], c[30005], d[30005];

int abs(int k){if(k < 0)return -k;return k;}

int main (){
     fin>>n>>m>>x>>y;
     for(int i=1; i<=m; i++){
          fin>>ii>>jj>>dd;
          v[ii].push_back( make_pair(jj,  dd) );
          v[jj].push_back( make_pair(ii, -dd) );
     }

     st=dr=1;
     c[st]=x;
     f[x]=1;
     while(st <= dr){
          nod=c[st];
          for(int i=0; i<v[nod].size(); i++){
               vcrt=v[nod][i].first ;
               dcrt=v[nod][i].second;

               if(f[vcrt] == 0){
                    f[vcrt] = 1;
                    c[++dr] = vcrt;
                    d[vcrt] = d[nod] + dcrt;
                    if(vcrt == y){
                         fout<<abs(d[y]);
                         return 0;
                    }
               }
          }
          st++;
     }

     return 0;
}