import sys
input = sys.stdin.readline

N,M=list(map(int,input().split()))
EDGE=[list(map(int,input().split())) for i in range(M)]

OK=0
NG=10**9+1

while NG>OK+1:
    mid=(OK+NG)//2

    E=[[] for i in range(N)]

    for x,y,h in EDGE:
        if h>=mid:
            E[x-1].append(y-1)
            E[y-1].append(x-1)

    USE=[0]*N
    USE[0]=1
    Q=[0]

    while Q:
        x=Q.pop()
        USE[x]=1
        for to in E[x]:
            if USE[to]==0:
                USE[to]=1
                Q.append(to)

    if USE[N-1]==1:
        OK=mid
    else:
        NG=mid

if OK==0:
    print("NaN")
else:
    print(OK)