#include<fstream>
#include <vector>
#include <bitset>
#include <queue>
using namespace std;
ifstream fin("sate.in");
ofstream fout("sate.out");

int n,m,X,Y;
queue<pair<int,int>> q;
bitset<30005> viz;
vector<pair<int,int>> L[30005];
int main() {
    int a,b,d;
    fin>>n>>m>>X>>Y;
    for (int i=1;i<=m;i++) {
        fin>>a>>b>>d;
        L[a].push_back(make_pair(b,d));
        L[b].push_back(make_pair(a,-d));
    }
    q.push(make_pair(X,0));
    viz[X]=1;
    while (!q.empty()) {
        auto[k,d]=q.front();
        q.pop();
        for (auto& w:L[k]) {
            if (viz[w.first]==0) {
                q.push(make_pair(w.first,d+w.second));
                viz[w.first] = 1;
                if (w.first == Y) {
                    fout<<d+w.second<<"\n";
                    return 0;
                }
            }
        }
    }


}