import sys
input = sys.stdin.readline

n, r = map(int, input().split(' '))
adj = [ [] for i in range(n+1) ]
vis = [False for i in range(n+1)]
prev = [-1 for i in range(n+1)]
bunnies = [-1 for i in range(n+1)]

for i in range(n-1):
    x, y = list(map(int, input().split(' ')))
    adj[x].append(y)
    adj[y].append(x)

b = []
for i in range(r):
    x = int(input())
    b.append(x)
    bunnies[x] = 0

#while len(b) > 0:
 #   node = b.pop(0)
   # for i in range(0, len(adj[node-1])):
    #    if bunnies[adj[node-1][i]-1] == False:
      #      bunnies[adj[node-1][i]-1] = bunnies[node-1] + 1
        #    b.append(bunnies[adj[node-1][i]])
while len(b) > 0:
    cur = b.pop(0)
    for neighbor in adj[cur]:
        if bunnies[neighbor] == -1:
            bunnies[neighbor] = bunnies[cur] + 1
            b.append(neighbor)

start, end = list(map(int, input().split(' ')))

queue = [start]
vis[start] = True
while len(queue) > 0:
    node = queue.pop(0)
    for neighbor in adj[node]:
        if vis[neighbor] == False:
            vis[neighbor] = True
            queue.append(neighbor)
            prev[neighbor] = node
cur, minrab = end, 1000000000
while cur != -1:
    minrab = min(minrab, bunnies[cur])
    cur = prev[cur]

print(minrab)