圆方树orz，参见猫的课件（apio和wc的）以及这里那里

#include <iostream>

#include <cstdio>

using namespace std;

typedef long long ll;

int n, m, uu, vv, oea[100005], loo[100005], dfn[100005], idx, sta[100005];

int siz[200005], val[200005], tot, sze, din, ont, cnt, hea[200005];

ll ans;

struct Edge{

	int too, nxt;

}odge[400005], edge[400005];

void add_odge(int fro, int too){

	odge[++ont].nxt = oea[fro];

	odge[ont].too = too;

	oea[fro] = ont;

}

void add_edge(int fro, int too){

	edge[++cnt].nxt = hea[fro];

	edge[cnt].too = too;

	hea[fro] = cnt;

}

void tarjan(int x){

	dfn[x] = loo[x] = ++idx;

	sta[++din] = x;

	siz[x] = 1;

	val[x] = -1;

	for(int i=oea[x]; i; i=odge[i].nxt){

		int t=odge[i].too;

		if(!dfn[t]){

			tarjan(t);

			loo[x] = min(loo[x], loo[t]);

			if(loo[t]>=dfn[x]){

				int p;

				tot++;

				add_edge(x, tot);

				do{

					p = sta[din--];

					val[tot]++;

					add_edge(tot, p);

					siz[tot] += siz[p];

				}while(p!=t);

				val[tot]++; siz[x] += siz[tot];

			}

		}

		else    loo[x] = min(loo[x], dfn[t]);

	}

}

void dfs(int x){

	if(x<=n)	ans += (ll)val[x] * (sze - 1);

	ans += (ll)val[x] * (sze - siz[x]) * siz[x];

	for(int i=hea[x]; i; i=edge[i].nxt){

		int t=edge[i].too;

		dfs(t);

		ans += (ll)val[x] * (sze - siz[t]) * siz[t];

	}

}

int main(){

	cin>>n>>m;

	tot = n;

	for(int i=1; i<=m; i++){

		scanf("%d %d", &uu, &vv);

		add_odge(uu, vv);

		add_odge(vv, uu);

	}

	for(int i=1; i<=n; i++)

		if(!dfn[i]){

			tarjan(i);

			sze = siz[i];

			dfs(i);

		}

	cout<<ans<<endl;

	return 0;

}