#include <fstream>
#include <queue>
#include <cstring>
#include <climits>
using namespace std;
ifstream cin("sate.in");
ofstream cout("sate.out");
int n,m,x,y,d[30005];
vector<pair<int,int>>l[30005];
queue<int>q;
int main()
{
    cin>>n>>m>>x>>y;
    for(int i=1;i<=m;i++)
    {
        int a,b,c;
        cin>>a>>b>>c;
        l[a].push_back({b,c});
        l[b].push_back({a,-c});
    }
    q.push(x);
    d[x]=0;
    while(!q.empty())
    {
        int nod=q.front();
        for(int i=0;i<l[nod].size();i++)
        {
            int vecin=l[nod][i].first;
            if(d[vecin]==0)
            {
                d[vecin]=d[nod]+l[nod][i].second;
                q.push(vecin);
            }
        }
        q.pop();
    }
    cout<<d[y];
    return 0;
}