#include <fstream>
#include <vector>
#include <queue>
#include <climits>
using namespace std;
int n, m, x, y, i, j, a, b, d, l[30001];
bool exp[30001];
ifstream fin ("sate.in");
ofstream fout ("sate.out");
vector < pair <int, int> > adj[30001];
queue <int> q;

void BFS (int s)
{
    int x=0;
    l[s]=0;
    for (int i=2;i<=n;i++) {
        l[i]=INT_MAX;
    }
    exp[s]=1;
    q.push(s);
    while (!q.empty()) {
        x=q.front();
        q.pop();
        for (int i=0;i<adj[x].size();i++) {
            if (!exp[adj[x][i].first]) {
                exp[adj[x][i].first]=1;
                l[adj[x][i].first]=l[x]+adj[x][i].second;
                q.push(adj[x][i].first);
            }
        }
    }
}

int main () {
    fin>>n>>m>>x>>y;
    for (i=1;i<=m;i++) {
        fin>>a>>b>>d;
        adj[a].push_back({b,d});
        adj[b].push_back({a,-d});
    }

    BFS(x);
    fout<<l[y];
    return 0;
}