#include <bits/stdc++.h>
#define DIM 30005
using namespace std;
ifstream fin ("sate.in");
ofstream fout ("sate.out");

    vector<pair<int,int>>g[DIM];
    int d[DIM],v[DIM];
    int n,m,a,b,c,x,y;
    queue<int> q;
void bfs()
{
    q.push(x);v[x]=1;
    while(!q.empty())
    {
        int nod=q.front();
        q.pop();
        for(auto w : g[nod])
        {
           int vecin=w.first;
           int cost=w.second;
           if(v[vecin]==0)
           {
            q.push(vecin);
            v[vecin]=1;
            if(vecin<nod)
                d[vecin]=d[nod]-cost;
            else if(vecin>nod)
                d[vecin]=d[nod]+cost;
            if(vecin==y)
                {fout<<d[y];return;}
            }
        }
    }
}

int main()
{
    fin >>n >>m >>x>>y;
    for(int i=1;i<=m;++i)
    {
        fin>>a>>b>>c;
        g[a].push_back({b,c});
        g[b].push_back({a,c});
    }
    bfs();

    return 0;
}