#include <fstream>
#include <vector>

using namespace std;

ifstream cin ("sate.in");
ofstream cout ("sate.out");

int n, m, a, b, x, y, z;

vector <pair <int, int>> g[30005];
int dist[30005], viz[30005];

void dfs(int nod) {
  viz[nod] = 1;
  for(auto &fiu : g[nod]) {
    if(!viz[fiu.first]) {
      dist[fiu.first] = dist[nod] + fiu.second;
      dfs(fiu.first);
    }
  }
}

int main() {
  cin >> n >> m >> a >> b;
  for(; m; m--) {
    cin >> x >> y >> z;
    g[x].push_back({y, z});
    g[y].push_back({x, -z});
  }
  dfs(a);
  cout << dist[b];
  return 0;
}