#include <iostream>
#include <fstream>
#include <vector>
#include <queue>
using namespace std;


ifstream fin("sate.in");
ofstream fout("sate.out");


int n,m,x,y,ii,jj,d;
int s[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>>ii>>jj>>d;
        v[ii].push_back({jj,d});
        v[jj].push_back({ii,-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(s[l]==0)
            {
                q.push(l);
                s[l]=s[k]+v[k][i].second;
            }
        }
    }

    fout<<s[y];
    return 0;
}