#include <fstream>
#include<vector>
#include<queue>
using namespace std;
ifstream fin("sate.in");
ofstream fout("sate.out");
struct elem
{
    int nod,dist;
};
vector<elem>a[30001];
queue<int>q;
int n,m,x,y,z,d[30001],st,fi,k,nod,dist;
bool viz[30001];
int main()
{
    fin>>n>>m>>st>>fi;
    for(int i=1;i<=m;i++)
    {
        fin>>x>>y>>z;
        a[x].push_back({y,z});
        a[y].push_back({x,z});
    }
    d[st]=0;
    q.push(st);
    viz[st]=1;
    while(!q.empty())
    {
        k=q.front();
        q.pop();
        for(int i=0;i<a[k].size();i++)
        {
            nod=a[k][i].nod;
            dist=a[k][i].dist;
            if(viz[nod]==0)
            {
                viz[nod]=1;
                if(nod>k)
                    d[nod]=d[k]+dist;
                else
                    d[nod]=d[k]-dist;
               q.push(nod);
            }
        }
    }
    fout<<d[fi];
    return 0;
}