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