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

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

const int nmax = 30001;

vector<pair<int, int>> graf[nmax];
int distanta[nmax];
bool vizitat[nmax];

int main(){
    int n, m, x, y;
    fin >> n >> m >> x >> y;

    for (int i = 0; i < m; i++){
        int a, b, d;
        fin >> a >> b >> d;
        graf[a].push_back({b, d});
        graf[b].push_back({a, -d});
    }

    queue<int> q;

    q.push(x);
    vizitat[x] = true;
    distanta[x] = 0;

    while (!q.empty()){
        int nod = q.front();
        q.pop();
        for (auto vecin : graf[nod]){
            int urmator = vecin.first;
            int d = vecin.second;
            if (!vizitat[urmator]){
                vizitat[urmator] = true;
                distanta[urmator] = distanta[nod] + d;
                q.push(urmator);
            }
        }
    }
    if (distanta[y] < 0)
        fout << -distanta[y];
    else
        fout << distanta[y];
    return 0;
}