#include <fstream>
#define x first
#define y second
#define vecin nodes[nod][i].x
#define dim 30001
#include <vector>
#include <queue>
using namespace std;
ifstream fin("sate.in");
ofstream fout("sate.out");
queue <int> q;
vector < pair <int,int> > nodes[dim];
int f[dim],i,m,x,y,a,b,c,nod,n;
int main()
{
    fin>>n>>m>>x>>y;
    for(i=1;i<=m;i++)
    {
        fin>>a>>b>>c;
        nodes[a].push_back(make_pair(b,c));
        nodes[b].push_back(make_pair(a,c));
    }
    q.push(x);
    while(!q.empty()&&!f[y])
    {
        nod=q.front();
        for(i=0;i<nodes[nod].size();i++)
        {
            if(!f[vecin])
            {
                if(vecin>=nod)
                    f[vecin]=f[nod]+nodes[nod][i].y;
                else
                    f[vecin]=f[nod]-nodes[nod][i].y;
                q.push(vecin);
            }
        }
        q.pop();
    }
    fout<<f[y];
    return 0;
}