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())
s -= 1
t -= 1
p = [int(i) for i in input().split()]
pd = {}
for ind, i in enumerate(p):
    if i not in pd:
        pd[i] = []
    pd[i].append(ind)

e = [[] for _ in range(n)]
for _ in range(m):
    a, b = [int(i) - 1 for i in input().split()]
    e[a].append(b)
    e[b].append(a)


uf = UnionFind(n)

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

for p_key in sorted(pd.keys(), reverse=True):
    a = []
    for i in pd[p_key]:
        if i == s:
            a.append(0)
        else:
            aa = -float("inf")
            for j in e[i]:
                if used[j]:
                    aa = max(aa, dp[uf.get_root(j)])
            a.append(aa+1)

    for i in pd[p_key]:
        used[i] = True
        for j in e[i]:
            if used[j]:
                uf.merge(i, j)

    for i, aa in zip(pd[p_key], a):
        r = uf.get_root(i)
        dp[r] = max(dp[r], aa)


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