N,M=map(int,input().split())
G=[[] for i in range(N)]
for i in range(M):
  a,b,c=map(int,input().split())
  G[a-1].append((b-1,c))
  G[b-1].append((a-1,c))
l=0
r=10**9
while True:
  if l==r:
    break
  m=(l+r+1)//2
  from collections import deque
  S=deque()
  d=[0]*N
  d[0]=1
  S.append(0)
  while S:
    x=S.pop()
    for B in G[x]:
      y,c=B[:]
      if c>=m:
        if d[y]==0:
          d[y]=1
          S.append(y)
  if d[N-1]==1:
    l=m
  else:
    r=m-1
if l==0:
  print('NaN')
else:
  print(l)