E

考察树的直径。 先随便取个点，搜出最距离此点最远节点，再以这个节点为起点，再搜一遍，最后重复一遍。搜的过程中记录一下距离是否大于2.

#include<bits/stdc++.h>
using namespace std;
const int N = 2e5 + 5;
int n;
bool vis[N];
struct node {
    int to, next;
}edge[N];
int head[N], cnt;
int ans;
int mxlen;
int nod;

void add(int u, int v)//前向星记录边
{
    edge[++cnt].to = v;
    edge[cnt].next = head[u];
    head[u] = cnt;
}

void dfs(int now, int fa, int len, int hea)
{
    if (len > 2) { vis[hea] = 1; vis[now] = 1; }
    if (mxlen < len) { mxlen = len; nod = now; }

    for (int i = head[now]; i; i = edge[i].next)
    {
        int x = edge[i].to;
        if (x != fa) { dfs(x, now, len + 1, hea); }
    }
}

int main(void)
{
    ios::sync_with_stdio(false), cin.tie(0), cout.tie(0);
    cin >> n;
    for (int i = 1; i < n; i++)
    {
        int u, v;
        cin >> u >> v;
        add(u, v);
        add(v, u);
    }

    dfs(1, 0, 0, 1);
    dfs(nod, 0, 0, nod);
    dfs(nod, 0, 0, nod);
    for (int i = 1; i <= n; i++)
        if (!vis[i])ans++;
    cout << ans;
}