#include <bits/stdc++.h>

using namespace std;
ifstream fin("sate.in");
ofstream fout("sate.out");
int n,m,x,y,a,b,d,dim[300005];
vector<pair<int,int>>v[30005];
queue<int>q;
int main()
{
    fin>>n>>m>>x>>y;
    for(int i=1;i<=m;i++)
    {
        fin>>a>>b>>d;
        v[a].push_back({b,d});
        v[b].push_back({a,-d});
    }
    q.push(x);
    while(!q.empty())
    {
        int k=q.front();
        q.pop();
        for(int i=0;i<v[k].size();i++)
        {
            int l=v[k][i].first;
            if(dim[l]==0)
            {
                q.push(l);
                dim[l]=dim[k]+v[k][i].second;
            }
        }
    }
    fout<<dim[y];
    return 0;
}