# 制約違反チェック

class UnionFind:
    def __init__(self, n):
        self.parent = [-1] * n
        self.rank = [1] * n
        self.count = [1] * n

    def get_root(self, i):
        while self.parent[i] != -1:
            i = self.parent[i]
        return i

    def is_same_tree(self, i, j):
        return self.get_root(i) == self.get_root(j)

    def merge(self, i, j):
        i = self.get_root(i)
        j = self.get_root(j)
        if i == j:
            return

        if self.rank[i] > self.rank[j]:
            self.parent[j] = i
            self.rank[i] = max(self.rank[i], self.rank[j] + 1)
            self.count[i] += self.count[j]
        else:
            self.parent[i] = j
            self.rank[j] = max(self.rank[j], self.rank[i] + 1)
            self.count[j] += self.count[i]


n, m, s, t = map(int, input().split())
assert 2<=n<=10**5 and 1<=m<=10**5, 1<=s<=n and 1<=t<=n
assert s != t

s -= 1
t -= 1
p = [(int(i), ind) for ind, i in enumerate(input().split())]
p.sort(reverse=True)
assert 1<=p[0][0] <= 10**9 and 1<=p[-1][0] <= 10**9

ss = set()
e = [[] for _ in range(n)]
for _ in range(m):
    a, b = [int(i) - 1 for i in input().split()]
    assert a<b
    e[a].append(b)
    e[b].append(a)
    assert (a,b) not in ss
    ss.add((a,b))


uf = UnionFind(n)

dp = [-float("inf")] * n
used = [False] * n

for _, ind in p:
    used[ind] = True
    if ind == s:
        a = 0
    else:
        a = -float("inf")
        for j in e[ind]:
            if used[j]:
                a = max(a, dp[uf.get_root(j)])
        a += 1

    for j in e[ind]:
        if used[j]:
            uf.merge(ind, j)

    dp[uf.get_root(ind)] = a

print(dp[uf.get_root(t)])