# include <fstream>
# include <vector>
# include <queue>
# define DIM 30010
# define f first
# define s second
using namespace std;
ifstream fin("sate.in");
ofstream fout("sate.out");
vector<pair<int,int> > Lista[DIM];
queue<int> q;
int Marcat[DIM],n,m,i,x,y,a,b,cost;
void bfs(){
    q.push(1);
    Marcat[1]=1;
    while(!q.empty()){
        int nc=q.front();
        q.pop();
        for(int i=0;i<Lista[nc].size();i++){
            int nv=Lista[nc][i].f;
            int cost=Lista[nc][i].s;
            if(Marcat[nv]==0){
                Marcat[nv]=Marcat[nc]+cost;
                q.push(nv);
            }
        }
    }
}
int main () {
    fin>>n>>m>>x>>y;
    if(x>y)
        swap(x,y);
    for(i=1;i<=m;i++){
        fin>>a>>b>>cost;
        Lista[a].push_back(make_pair(b,cost));
        Lista[b].push_back(make_pair(a,-cost));
    }
    bfs();
    fout<<Marcat[y]-Marcat[x]<<"\n";
    return 0;
}