#include <iostream>
#include <cstring>
#include <fstream>
#include <vector>
#include <queue>

#define N 30005

using namespace std;

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

vector <int> v[N], d[N], dis(N, 0);
vector <bool> viz(N, 0);
queue <short> q;

int main()
{
    int n, m, x, y, a, b, l;
    fin>>n>>m>>x>>y;
    while(m--)
    {
        fin>>a>>b>>l;
        v[a].push_back(b);
        v[b].push_back(a);
        d[a].push_back(l);
        d[b].push_back(-l);
    }

    viz[x] = 1;

    q.push(x);
    while(!q.empty() && !viz[y])
    {
        int z = q.front();
        q.pop();
        for(unsigned i=0; i<v[z].size(); i++)
            if(!viz[v[z][i]])
            {
                viz[v[z][i]] = 1;
                dis[v[z][i]] = dis[z] + d[z][i];
                q.push(v[z][i]);
            }
    }

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