//#include <iostream>
#include <fstream>
#include <vector>
#include <queue>
using namespace std;
ifstream cin("sate.in");
ofstream cout("sate.out");
vector<pair<int,int> > v[30005];
queue<pair<int,int> > q;
int d[30005];
int main()
{
    int n,m,x,y,a,b,di;
    cin>>n>>m>>x>>y;
    for(int i=1;i<=m;i++){
        cin>>a>>b>>di;
        v[a].push_back(make_pair(b,di));
        v[b].push_back(make_pair(a,-di));
    }
    for(int i=1;i<=n;i++){
        d[i]=30000000;
    }
    q.push(make_pair(x,0));
    while(q.empty()==false){
        pair<int,int> a=q.front();
        q.pop();
        if(d[a.first]!=30000000)
            continue;
        d[a.first]=a.second;
        for(auto u:v[a.first]){
            q.push(make_pair(u.first,a.second+u.second));
        }
    }
    cout<<d[y];
    return 0;
}