import heapq
N,M=map(int,input().split())
H=list(map(int,input().split()))
G=[[]for _ in range(N)]
for _ in range(M):
 a,b=map(int,input().split())
 a-=1;b-=1
 G[a].append(b);G[b].append(a)
def b(v):
 d=[-1]*N
 s=[0]*N
 d[v]=0
 q=[(H[v],v)]
 while q:
  _,v=heapq.heappop(q)
  if s[v]==1:continue
  s[v]=1
  for n in G[v]:
   if H[v]<H[n] and d[n]<d[v]+1:
    d[n]=d[v]+1
    heapq.heappush(q,(H[n],n))
 return d
x=b(0);y=b(N-1)
a=-1
for i in range(N):
 if x[i]==-1 or y[i]==-1:
  continue
 a=max(a,x[i]+y[i]+1)
print(a)