#include <fstream>
#include <vector>
#define Max 100100
using namespace std;

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

int n, m, i, j, k, ok, minim, maxim,z;
vector < int > v[Max];
int vizit[Max], dist[Max], p, u, x, y;

void bfs(int val){
    vizit[val] = 1;
    for(int i = 0; i < v[val].size(); i += 2){
        if(vizit[v[val][i]] == 0){
            dist[v[val][i]] = dist[val] + v[val][i + 1];
            bfs(v[val][i]);
        }
    }
    return;
}

void read(){
    fin >> n >> m >> p >> u;
    for(i = 1; i <= m; i ++){
        fin >> x >> y >> z;
        v[x].push_back(y);
        v[x].push_back(z);
        v[y].push_back(x);
        v[y].push_back(-z);
    }
    return;
}

int main(){
    read(); bfs(p);
    fout << dist[u];
    return 0;
}