#include <fstream>
#include <vector>

using namespace std;

vector < pair < int, int > > v[100205];
int viz[30005],dist[30005];
int myabs(int b)
{
    if(b<0)
       b=-b;
    return b;
}
void dfs(int nod)
{
    viz[nod]=1;
    for(int i=0;i<v[nod].size();i++)
    {
        if(viz[v[nod][i].first]==0)
        {
            dist[v[nod][i].first]=dist[nod]+v[nod][i].second;
            dfs(v[nod][i].first);
        }
    }
}

int main()
{

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