I,R=input,range
J=lambda:map(int,I().split())
import heapq
N,M=J()
E=[[]for i in R(N+1)]
for _ in[0]*M:u,v,h=J();E[u]+=[[v,-h]];E[v]+=[[u,-h]]
z=-10**9
W=[0]*(N+1)
W[1]=z
S=[[z,1]]
while S:
	w,i=heapq.heappop(S)
	if w>W[i]:continue
	for j,v in E[i]:
		v=max(w,v)
		if v<W[j]:W[j]=v;heapq.heappush(S,[v,j])
print(["NaN",-W[N]][W[N]<0])