even_tree.cc

#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#include <set>
#include <map>
#include <queue>
using namespace std;

map<int, vector<int> > graph;

bool count_down(int start){
    queue<int> tovisit;
    set<int> visited;
    tovisit.push(start);
    while(tovisit.size()){
        int node = tovisit.front();
        visited.insert(node);
        tovisit.pop();
        for(int i=0; i<graph[node].size(); ++i){
            int sib = graph[node][i];
            if(!visited.count(sib)){
                tovisit.push(sib);
            }
        }
    }

    return visited.size()%2 == 0 && visited.size() >= 2;
}

int main() {
    int M,N;
    cin >> M >> N;
    for(int i=0; i<N; ++i){
        int u, v;
        cin >> u >> v;
        graph[v].push_back(u);
    }
    int ans = 0;
    for(int i=1; i<=M; ++i){
        ans += count_down(i) ? 1 : 0;
    }
    printf("%d\n", ans - 1);
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */   
    return 0;
}