n, m, s, t = map(int, input().split())

graph = [[] for _ in range(n + 1)]
in_degree = [0] * (n + 1)

for _ in range(m):
    u, v, weight = map(int, input().split())
    graph[u].append((v, weight))
    in_degree[v] += 1

queue = []
for i in range(1, n + 1):
    if in_degree[i] == 0:
        queue.append(i)

order = []
head = 0

while head < len(queue):
    current = queue[head]
    head += 1
    order.append(current)
    
    for neighbor, weight in graph[current]:
        in_degree[neighbor] -= 1
        if in_degree[neighbor] == 0:
            queue.append(neighbor)

dist = [10**18] * (n + 1)
dist[s] = 0

for node in order:
    if dist[node] != 10**18:
        for neighbor, weight in graph[node]:
            if dist[node] + weight < dist[neighbor]:
                dist[neighbor] = dist[node] + weight

if dist[t] == 10**18:
    print("Unreachable")
else:
    print(dist[t])