CF1000E We Need More Bosses

#include<bits/stdc++.h>
#define r register
using namespace std;
const int N=3e5+5;
inline int read() {
    int X=0,w=0; char ch=0;
    while(!isdigit(ch)) {w|=ch=='-';ch=getchar();}
    while(isdigit(ch)) X=(X<<3)+(X<<1)+(ch^48),ch=getchar();
    return w?-X:X;
}
int to[2*N],nxt[2*N],head[N],tot=1;
int dfn[N],low[N],time_,id[N],k;
bool cut[2*N];
inline void add(int u,int v) {
    to[++tot]=v,nxt[tot]=head[u],head[u]=tot;
}
inline void tarjan(int u,int in_edge) {
    dfn[u]=low[u]=++time_;
    for (r int i=head[u];i;i=nxt[i]) {
        int v=to[i];
        if (!dfn[v]) {
            tarjan(v,i);
            low[u]=min(low[u],low[v]);
            if (low[v]>dfn[u])
                cut[i]=cut[i^1]=1;
        }
        else 
          if (i!=(in_edge^1))
            low[u]=min(low[u],dfn[v]);
    }
}
void dfs(int u) {
    id[u]=k;
    for (r int i=head[u];i;i=nxt[i]) {
        int v=to[i];
        if (id[v] || cut[i]) continue;
        dfs(v);
    }
}
vector<int> a[N];
bool vis[N];
int s_, id_;
void dfs_ (int u,int s) {
    vis[u]=1;
    if (s>s_) s_=s,id_=u;
    for (r int i=0;i<a[u].size();i++)
        if (!vis[a[u][i]])
            dfs_(a[u][i],s+1);
}
int main() {
    int n=read(),m=read();
    for (int i=1;i<=m;i++) {
        int u=read(),v=read();
        add(u,v),add(v,u); 
    }
    for (r int i=1;i<=n;i++)
        if (!dfn[i]) tarjan(i,0);
    for (r int i=1;i<=n;i++)
        if (!id[i]) k++,dfs(i);
    for (r int i=1;i<=n;i++)
        for (r int j=head[i];j;j=nxt[j])
            if (id[i]!=id[to[j]])
                a[id[i]].push_back(id[to[j]]);
    dfs_(1,1);
    memset(vis,0,sizeof(vis));
    dfs_(id_,1);
    printf("%d
",s_-1);
    return 0;
}