#include <fstream>
#include <vector>
using namespace std;
struct muchie{
  int x,c;
};
vector < muchie > v[30001];
int a,b,n,m,x,y,k1,k2,q[90000001],viz[30001],j,S,i,d;
void coada(int x){
    int p,u,i,l;
    p=u=1;
    q[p]=x;
    viz[x]=1;
    while(p<=u){
        l=q[p];
        p++;
        for(i=0;i<v[l].size();i++)
            if(viz[v[l][i].x]==0){
                u++;
                q[u]=v[l][i].x;
                if(v[l][i].x<l)
                    viz[v[l][i].x]=viz[l]-v[l][i].c;
                else
                    viz[v[l][i].x]=viz[l]+v[l][i].c;
            }
    }

}
int main()
{
    ifstream cin("sate.in");
    ofstream cout("sate.out");
    cin>>n>>m>>x>>y;
    for(i=1;i<=m;i++){
       cin>>a>>b>>d;
       v[a].push_back({b,d});
       v[b].push_back({a,d});
    }
    S=0;
 coada(x);
 viz[y]--;
    cout<<viz[y];
    return 0;
}