#include<iostream>
#include<cstring>
#include<algorithm>
#include<queue>
#include<map>
#include<stack>
#include<cmath>
#include<vector>
#include<set>
#include<cstdio>
#include<string>
#include<deque>
using namespace std;
typedef long long LL;
#define eps 1e-8
#define INF 0x3f3f3f3f
#define maxn 1000005
int head[maxn],pre[maxn],dis[maxn],vis[maxn];
int n,m,k,t,cnt,flag,max1,point;
//max1记录每次bfs的最大距离，point记录离树根最远的点
struct node{int v,w,next;
}edge[maxn*20];
void init(){memset(head,-1,sizeof(head));for(int i=1;i<=n;i++)pre[i]=i;cnt=flag=0;
}
int find(int a){if(pre[a]==a)return a;return pre[a]=find(pre[a]);
}
void add(int u,int v,int w){edge[++cnt].v=v;edge[cnt].w=w;edge[cnt].next=head[u];head[u]=cnt;
}
void bfs(int u){//bfs找以点u为子树的所有点里面离点u最远的点和这个最远的距离 queue<int>q;dis[u]=0;q.push(u);while(!q.empty()){u=q.front();q.pop();vis[u]=true;for(int i=head[u];i!=-1;i=edge[i].next){int v=edge[i].v;int w=edge[i].w;if(!vis[v]){dis[v]=dis[u]+w;q.push(v);if(max1<dis[v]){//更新最远的点和最大距离 max1=dis[v];point=v;}}}}}
int main()
{while(scanf("%d%d",&n,&m)!=EOF){init();int u,v,w;for(int i=1;i<=m;i++){scanf("%d%d%d",&u,&v,&w);if(flag)continue;int x=find(u);int y=find(v);if(x==y)//并查集判断是否有环 flag=1;else{pre[x]=y;add(u,v,w);add(v,u,w);}}if(flag){printf("YES\n");continue;}memset(vis,0,sizeof(vis));memset(dis,0,sizeof(dis));stack<int>ss;//因为可能给的数据是森林，不一定是只有一棵树，所以我把每颗树里面//离树根最远的点存进栈里面 for(int i=1;i<=n;i++){if(vis[i])continue;max1=0;bfs(i);ss.push(point);}int ans=0;memset(vis,0,sizeof(vis));memset(dis,0,sizeof(vis)); while(!ss.empty()){//再用栈里面的点来进行第二次bfs找到树的直径的另外一个点和树的直径 point=ss.top();ss.pop();max1=0;bfs(point);ans=max(max1,ans);}printf("%d\n",ans);}return 0;
}

#include<iostream>
#include<cstring>
#include<algorithm>
#include<queue>
#include<map>
#include<stack>
#include<cmath>
#include<vector>
#include<set>
#include<cstdio>
#include<string>
#include<deque>
using namespace std;
typedef long long LL;
#define eps 1e-8
#define INF 0x3f3f3f3f
#define maxn 100005
int n,m,k,t,ans,flag,max1,cnt,point;
int dis[maxn],vis[maxn],head[maxn],pre[maxn];
struct node{int v,next,w;
}edge[maxn*20];
void init(){memset(head,-1,sizeof(head));for(int i=1;i<=n;i++)pre[i]=i;cnt=flag=0;max1=0;
}
int find(int a){if(pre[a]==a)return a;return pre[a]=find(pre[a]);
}
void add(int u,int v,int w){edge[++cnt].v=v;edge[cnt].w=w;edge[cnt].next=head[u];head[u]=cnt;
}
void dfs(int u){//求以u为根节点的子树中离点u最远的点已经最大距离 vis[u]=true;for(int i=head[u];i!=-1;i=edge[i].next){int v=edge[i].v;int w=edge[i].w;if(!vis[v]){dis[v]=max(dis[v],dis[u]+w);if(dis[v]>max1){point=v;max1=dis[v];}dfs(v);}}
}
int main()
{while(scanf("%d%d",&n,&m)!=EOF){init();int u,v,w;for(int i=1;i<=m;i++){scanf("%d%d%d",&u,&v,&w);if(flag)continue;int x=find(u);int y=find(v);if(x==y)flag=1;else{pre[x]=y;add(u,v,w);add(v,u,w);}}if(flag){printf("YES\n");continue;}int ans=0;memset(vis,0,sizeof(vis));queue<int>q;//用队列来存第一次dfs找出的所有点 for(int i=1;i<=n;i++){if(vis[i])continue;dfs(i);q.push(point);max1=0;}memset(dis,0,sizeof(dis));memset(vis,0,sizeof(vis));while(!q.empty()){point=q.front();q.pop();max1=0;dfs(point);ans=max(ans,max1);}printf("%d\n",ans);}return 0;
}