m=lambda x:[*map(int,x.split())]
A,B,*C=open(0)
N,M=m(A)
H=0,*m(B)
G=[[]for i in range(N+1)]
for s in C:a,b=m(s);G[a]+=b,;G[b]+=a,
D=[]
for c in 1,N:
 S=[-1e9]*-~N;S[c]=0;Q=[c]
 for t in Q:
  for n in G[t]:
   if H[n]>H[t]and S[n]<=S[t]:S[n]=S[t]+1;Q+=n,
 D+=S,
print(max(-2,*map(sum,zip(*D)))+1)