from collections import deque

with open("sate.in") as f:
  date = list(map(int, f.read().split()))

n, m, x, y = date[0], date[1], date[2], date[3]

vecini = [[] for _ in range(n+1)]

idx = 4
for _ in range(m):
  l = date[idx]
  idx += 1
  r = date[idx]
  idx += 1
  d =  date[idx]
  idx += 1
  vecini[l].append((r, d))
  vecini[r].append((l,-d))

dist = [0] * (n+1)
viz = [False] * (n+1)
viz[x] = True

coada = deque([x])
while coada and not viz[y]:
  l = coada.popleft()
  for v, cost in vecini[l]:
    if not viz[v]:
      viz[v] = True
      dist[v] = dist[l]+cost
      coada.append(v)

with open("sate.out", "w") as g:
  g.write(str(abs(dist[y])) + "\n")