luogu_1330 封锁阳光大学

#include <cstdio>
#include <vector>
#include <iostream>
using namespace std;
const int maxn=100010;
int n,m,sum,sum1,sum2;
vector<int> G[maxn];
int color[maxn],color2[maxn];

bool dfs(int u){
    for(int i=0;i<G[u].size();i++){
        int v=G[u][i];
        if(!color[v]){
            color[v]=3-color[u];
            if(color[v]==2)sum2++;
            if(color[v]==1)sum1++;
            if(!dfs(v))return false;
        }
        else if(color[u]==color[v])return false;
    }
    return true;
}

bool solve(){
    for(int i=1;i<=n;i++)
        if(!color[i]){
            sum1=sum2=0;
            color[i]=1; sum1++;
            if(!dfs(i))return false;
            else sum+=min(sum1,sum2);
        }
    return true;
}

int main(){
    scanf("%d%d",&n,&m);
    while(m--){
        int u,v;
        scanf("%d%d",&u,&v);
        G[u].push_back(v); G[v].push_back(u);
    }
    if(solve())printf("%d\n",sum);
    else puts("Impossible");
    return 0;
}