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

using namespace std;

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

const int NMAX=30005;

vector<pair<int,int>>v[NMAX];
queue<int>q;

int dist[NMAX];
bool viz[NMAX];

void bfs(int x)
{
    dist[x]=0;
    viz[x]=1;
    q.push(x);
    while(!q.empty())
    {
        int p=q.front();
        for(auto i:v[p])
        {
            if(!viz[i.first])
            {
                viz[i.first]=1;
                if(i.first>p)
                    dist[i.first]=dist[p]+i.second;
                else
                    dist[i.first]=dist[p]-i.second;
                q.push(i.first);
            }
        }
        q.pop();
    }
}

int main()
{
    int n,m,st,nd,x,y,c;
    fin>>n>>m>>st>>nd;
    while(m--)
    {
        fin>>x>>y>>c;
        v[x].push_back(make_pair(y,c));
        v[y].push_back(make_pair(x,c));
    }
    bfs(st);
    fout<<dist[nd];
    return 0;
}