#include <bits/stdc++.h>
#define inf 10000000
using namespace std;
ifstream f("sate.in");
ofstream g("sate.out");
int n,m,x,y;
vector<pair<int,int > > vecini[30005];
bool viz[30005];
int dist[30005];
queue<int> q;
int main()
{
f>>n>>m>>x>>y;
for(int i=1;i<=m;i++)
{
    int a,b,cost;
    f>>a>>b>>cost;
    vecini[a].push_back({b,cost});
    vecini[b].push_back({a,-cost});

}
viz[x]=1;
q.push(x);
while(!q.empty())
{
    int node=q.front();
    for(int i=0;i<vecini[node].size();i++)
    {
        int newnode=vecini[node][i].first;
        if(viz[newnode]==0)
        {
            viz[newnode]=1;
            dist[newnode]=dist[node]+vecini[node][i].second;
            q.push(newnode);
            if(newnode==y)
            {
                g<<dist[y];
                return 0;
            }
        }
    }
    q.pop();

}

}