用bfs求出每个连通分支的层数，然后把层数/2加起来为什么不对啊。。求助大佬。代码如下:

#include<iostream>
#include<queue>
using namespace std;
const int N = 10005;
bool g[N][N];
int color[N];
int main() {
    int n, m, ans = 0;
    cin >> n >> m;
    for (int i = 1; i <= n; ++i)g[i][i] = 0;
    for (int i = 1; i <= m; ++i) {
        int a, b;
        cin >> a >> b;
        g[a][b] = g[b][a] = 1;
    }
    queue<int>q;
    for (int i = 1; i <= n; ++i) {
        if (color[i]) continue;
        int cnt1 = 1, cnt2 = 0, res = 0;
        q.push(i); color[i] = 1;
        while (!q.empty()) {
            int t = q.front();
            q.pop();
            for (int i = 1; i <= n; ++i) {
                if (color[i] || g[t][i] == 0)continue;
                q.push(i);
                color[i] = 3 - color[t];
            }
            --cnt1, ++cnt2;
            if (!cnt1)cnt1 = cnt2, cnt2 = 0, ++res;
        }
        ans += res/2;
    }
    for (int i = 1; i <= n; ++i) 
        for(int j=i+1;j<=n;++j)
            if (g[i][j] && color[i] == color[j]) {
                cout << "Impossible";
                return 0;
            }
    cout << ans;
}