找不同，兩棵樹中不同邊的數量即為所求

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

const int maxn = 1e5+50;
int n;
int t[maxn];

int main()
{
        int a, b, x, y;
        scanf("%d", &n);
        for (int i = 0; i < n-1; i ++) {
                scanf("%d%d", &a, &b);
                if (!t[a]) t[a] = b;
                else t[b] = a;
        }
        int cnt = 0;
        for (int i = 0; i < n-1; i ++) {
                scanf("%d%d", &a, &b);
                if (t[a] == b || t[b] == a) cnt ++;
        }
        printf("%d\n", n-cnt-1);
}