n,m=map(int,input().split())
*h,=map(int,input().split())
e=[[]for _ in range(n)]
for i in range(m):
 a,b=map(int,input().split())
 a-=1
 b-=1
 e[a].append(b)
 e[b].append(a)
o=sorted(range(n),key=lambda x:h[x])


t=[-n*2]*n
t[0]=0
for i in o:
 for v in e[i]:
  if h[v]<h[i]:t[i]=max(t[i],t[v]+1)


u=[-n*2]*n
u[n-1]=0
for i in o:
 for v in e[i]:
  if h[v]<h[i]:u[i]=max(u[i],u[v]+1)

a=max(t[i]+u[i]for i in range(n))
if a<0:
 print(-1)
else:
 print(a+1)