[洛谷P1330]封锁阳光大学

#include <cstdio>
#include <algorithm>
#include <cstdlib>
#define maxn 10010
#define maxm 100010
int head[maxn], cnt;
struct Edge {
	int to, nxt;
} e[maxm << 1];
inline void addE(int a, int b) {
	e[++cnt] = (Edge) {b, head[a]}; head[a] = cnt;
}

int n, m, ans;
int res[2];
int C[maxn];
void dfs(int u) {
	res[C[u]]++;
	for (int i = head[u]; i; i = e[i].nxt) {
		int v = e[i].to;
		if (!~C[v]) {
			C[v] = C[u] ^ 1;
			dfs(v);
		} else if (C[v] != (C[u] ^ 1)) {
			puts("Impossible");
			exit(0);
		}
	}
}
int main() {
	scanf("%d%d", &n, &m);
	for (int i = 0, a, b; i < m; i++) {
		scanf("%d%d", &a, &b);
		addE(a, b);
		addE(b, a);
	}
	__builtin_memset(C, -1, sizeof C);
	for (int i = 1; i <= n; i++) if (!~C[i]) {
		res[0] = res[1] = 0;
		C[i] = 1;
		dfs(i);
		ans += std::min(res[0], res[1]);
	}
	printf("%d\n", ans);
	return 0;
}