#include <fstream>
#include <queue>

using namespace std;

class Edge {
   public:
      int to;
      int cost;
      
      Edge(int _to = 0, int _cost = 0) :
         to(_to), cost(_cost) {}
};

const int N_MAX = 30001;

int n, m, _begin, _end;
vector<Edge> graph[N_MAX];
int dist[N_MAX];
bool visited[N_MAX];
queue<int> q;

int main() {
   ifstream f("sate.in");
   
   f >> n >> m >> _begin >> _end;
   while(m--) {
      int x, y, d;
      f >> x >> y >> d;
      graph[x].emplace_back(y, d);
      graph[y].emplace_back(x, -d);
   }
   dist[_begin] = 0;
   visited[_begin] = 1;
   q.push(_begin);
   while(!q.empty()) {
      int crt = q.front();
      q.pop();
      for(auto edge : graph[crt]) {
         if(!visited[edge.to]) {
            visited[edge.to] = 1;
            dist[edge.to] = dist[crt] + edge.cost;
            q.push(edge.to);
         }
      }
   }
   
   ofstream("sate.out") << dist[_end] << '\n';
   return 0;
}