#include <iostream>
#include <cstdio>
#include <vector>
#include <queue>
#define NMAX 30000
#define pii pair<int, int>

using namespace std;

int n, m, x, y;
int dist[NMAX + 5];
vector<pii> v[NMAX + 5];
queue<int> q;

void bfs(int start) {
  int crt;

  dist[start] = 1;
  q.push(start);
  while(!q.empty()) {
    crt = q.front();
    q.pop();
    for(pii next: v[crt])
      if(!dist[next.first]) {
        dist[next.first] = dist[crt] + next.second;
        if(dist[y]) {
          printf("%d", dist[y] - 1);
          return;
        }
        q.push(next.first);
      }
  }
}

int main() {
  freopen("sate.in", "r", stdin);
  freopen("sate.out", "w", stdout);
  int a, b, d;

  scanf("%d %d %d %d", &n, &m, &x, &y);
  for(int i = 1; i <= m; i++) {
    scanf("%d %d %d", &a, &b, &d);
    v[a].push_back({b, d});
    v[b].push_back({a, -d});
  }

  if(x == y)
    printf("0");
  else
    bfs(x);

  return 0;
}