#include<fstream>
#include<vector>
#include<deque>
using namespace std;
ifstream f("sate.in");
ofstream g("sate.out");
int n,m,inc,sf,i,x,y,d[30005],cost,nod,j,a[20005][20005];
vector<pair<int,int> >v[30005];
deque<int>q;
int main()
{
    f>>n>>m>>inc>>sf;

    for(i=1;i<=m;i++)
    {
        f>>x>>y>>cost;
        v[x].push_back(make_pair(y,cost));
        v[y].push_back(make_pair(x,cost));
    }
    d[inc]=0;
    q.push_back(inc);
    while(!q.empty())
    {
        nod=q.front();
        q.pop_front();
        for(i=0;i<v[nod].size();i++)
        {
            q.push_back(v[nod][i].first);
            if(nod<v[nod][i].first)
              d[v[nod][i].first]=v[nod][i].second+d[nod];
            else
              d[v[nod][i].first]=d[nod]-v[nod][i].second;
        }
        if(d[sf]!=0)
        {
            g<<d[sf];
            return 0;
        }
    }
    return 0;
}