The Flee Plan of Groundhog
dfs
相向而行就距離-3，時間+1
背道而馳就距離-1，時間+1
注意土撥鼠可以不走，特判就好

#include <bits/stdc++.h>
#define inf 2333333333333333
#define N 1000010
#define p(a) putchar(a)
#define For(i,a,b) for(int i=a;i<=b;++i)
//by war
//2020.8.8
using namespace std;
int n,s,t,x,y,ans;
int d[N];
struct node{
    int n;
    node 
 
*next;
}*e[N];

void in(int &x){
    int y=1;char c=getchar();x=0;
    while(c<'0'||c>'9'){if(c=='-')y=-1;c=getchar();}
    while(c<='9'&&c>='0'){ x=(x<<1)+(x<<3)+c-'0';c=getchar();}
    x*=y;
}
void o(int x){
    if(x<0){p('-');x=-x;}
    if(x>9)o(x/10);
    p(x%10
 
+'0');
}

void push(int x,int y){
    node *p;
    p=new node();
    p->n=y;
    if(e[x]==0)
        e[x]=p;
    else{
        p->next=e[x]->next;
        e[x]->next=p;
    }
}

void dfs1(int x,int fa,int deep){
    d[x]=deep;
    for(node *i=e[x];i;i=i->next){
        if(i->n==fa) continue
 
;
        dfs1(i->n,x,deep+1);
    }
}

void dfs2(int x,int fa,int deep){
    if(deep==t){
        s=x;
        return;
    }
    for(node *i=e[x];i;i=i->next){
        if(i->n==fa || d[i->n]>=d[x]) continue;
        dfs2(i->n,x,deep+1);
    }
}

void dfs3(int x,int fa,int dis,int t){
    if(dis<=0){
        ans=max(ans,t);
        return;
    }
    for(node *i=e[x];i;i=i->next){
        if(i->n==fa) continue;
        if(d[i->n]<d[x]){
            dfs3(i->n,x,dis-3,t+1);
        }else{
            dfs3(i->n,x,dis-1,t+1);
        }
    }
    if(dis&1) ans=max(ans,t+dis/2+1);
    else ans=max(ans,t+dis/2);
}

signed main(){
    in(n);in(t);
    For(i,1,n-1){
        in(x);in(y);
        push(x,y);
        push(y,x);
    }
    dfs1(n,n,0);
    dfs2(1,1,0);
    dfs3(s,s,d[s],0);
    o(ans);
    return 0;
}