## https://yukicoder.me/problems/no/3490

from collections import deque

def reachable(N, next_nodes, height):
    queue = deque()
    passed = [False] * N
    passed[0] = True
    queue.append(0)
    while len(queue) > 0:
        v = queue.popleft()
        for w, h in next_nodes[v]:
            if h >= height:
                if not passed[w]:
                    passed[w] = True
                    queue.append(w)
    return passed[N - 1]


def main():
    N, M = map(int, input().split())
    next_nodes = [[] for _ in range(N)]
    max_h = 0
    for _ in range(M):
        u, v, h = map(int, input().split())
        next_nodes[u - 1].append((v - 1, h))
        next_nodes[v - 1].append((u - 1, h))
        max_h = max(max_h, h)


    if not reachable(N, next_nodes, 1):
        print("NaN")
        return
    
    low = 1
    high = max_h
    while high - low > 1:
        mid = (high + low) // 2
        if reachable(N, next_nodes, mid):
            low = mid
        else:
            high = mid
    if reachable(N, next_nodes, high):
        print(high)
    else:
        print(low)
        





if __name__ == "__main__":
    main()