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

using namespace std;

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

vector <pair<int,int>> graf[30001];

stack <pair<int,int>> ms;

int n,m,st,fn,vf[30001],dist;

void citire()
{
    in>>n>>m>>st>>fn;
    for(int i=1;i<=m;i++)
    {
        int x,y,z;
        in>>x>>y>>z;
        graf[x].push_back({y,z});
        graf[y].push_back({x,z});
    }
}

void dfs()
{
    ms.push({st,0});
    vf[st]=1;
    while(!ms.empty())
    {
        int nod=ms.top().first;
        ms.pop();
        int nr=graf[nod].size();
        for(int i=0;i<nr;i++)
        {
            int nextnod=graf[nod][i].first,drum=graf[nod][i].second;
            if(vf[nextnod]==0)
            {
                vf[nextnod]=1;
                if(nextnod<nod)
                    dist-=drum;
                else
                    dist+=drum;
                ms.push({nextnod,drum});
            }
        }
    }
}

int main()
{
    citire();
    dfs();
    out<<dist;
    return 0;
}