#include<bits/stdc++.h>
using namespace std;

int main(){
  int n,m;
  cin>>n>>m;
  vector<vector<pair<int,int>>> g(n);
  for (int i=0;i<m;i++){
    int u,v,h;
    cin>>u>>v>>h;
    u--;v--;
    g[u].push_back({v,h});
    g[v].push_back({u,h});
  }
  int l=0,r=1e9+10;
  while (r-l>1){
    int mid=(l+r)/2;
    vector<bool> vis(n);
    queue<int> q;
    vis[0]=true;
    q.push(0);
    while (!q.empty()){
      int v=q.front();
      q.pop();
      for (auto [u,h]:g[v]){
        if (h<mid) continue;
        if (vis[u]) continue;
        vis[u]=true;
        q.push(u);
      }
    }
    if (vis[n-1]) l=mid;
    else r=mid;
  }
  if (l==0) cout<<"NaN\n";
  else cout<<l<<endl;
}