题意：

思路

#include <cstdio>
#include <vector>
#include <cstring>
#include <iostream>
using namespace std;
const int N = 1e4 + 10;
const int M = 1e6 + 10;
vector<int> G[N];
int n,now;
bool match[M];
int used[M];
bool Hungary(int u){
    for(int i = 0;i < G[u].size();i++){
        int v = G[u][i];
        if(used[v] != now){
            used[v] = now;
            if(!match[v] || Hungary(match[v])){
                match[v] = u;
                return 1;
            }
        }
    }
    return 0;
}
int main(){
    scanf("%d",&n);
    for(int i = 1,u,v;i <= n;i++){
        scanf("%d%d",&u,&v);
        G[u].push_back(i),G[v].push_back(i);
    }
    int ans = 0;
    for(int i = 1;i <= n;i++){
        now = i;
        if(Hungary(i)) ans++;
        else break;
    }
    printf("%d\n",ans);
    return 0;
}