/*#include <iostream>
#include <cmath>
#include <vector>
#include <algorithm>
#include <array>
#include <vector>
#include <deque>
#include <set>
#include <map>
#include <string>
#include <stack>
#include <queue>
#include <unordered_map>
#include <unordered_set>
*/

#include <bits/stdc++.h>
using namespace std;

#define int long long
#define pb push_back
#define mp make_pair
#define F first
#define S second
#define FOR(i,a,b) for(int (i)=(a);(i)<(b);(i)++)
#define rep(i,n) FOR(i,0,n)
#define ALL(a) (a).begin(),(a).end()
#define RALL(a) (a).rbegin(),(a).rend()
#define SORT(c) sort((c).begin(),(c).end())

typedef long long ll;
const ll INF = LLONG_MAX - 100;
const ll mod = 1e9 + 7;
const int MAX_N = 5e5 + 5;
int dx[] = { -1LL,0,1LL,0 }, dy[] = { 0,1LL,0,-1LL };
vector<ll> prime;

ll inv[MAX_N], fac[MAX_N];

template <class T = ll> T in() { T x; cin >> x; return (x); }
inline ll GCD(ll a, ll b) { ll c; while (b != 0) { c = a % b; a = b; b = c; }return a; }
inline ll LCM(ll a, ll b) { return a * b / GCD(a, b); }
inline ll POW(ll a, ll b) { ll c = 1LL; while (b > 0) { if (b & 1LL) { c = a * c%mod; }a = a * a%mod; b >>= 1LL; }return c; }
inline void _nCr() { fac[0] = 1LL; for (int i = 1LL; i < MAX_N; i++) { fac[i] = fac[i - 1LL] * i%mod; }for (int i = 0; i < MAX_N; i++) { inv[i] = POW(fac[i], mod - 2); } }
inline ll nCr(ll n, ll r) { return (fac[n] * inv[r] % mod)*inv[n - r] % mod; }
inline void PRI(ll n) { bool a[n + 1LL]; for (int i = 0; i < n + 1LL; i++) { a[i] = 1LL; }for (int i = 2; i < n + 1LL; i++) { if (a[i]) { prime.pb(i); ll b = i; while (b <= n) { a[b] = 0; b += i; } } } }
void chmin(int &a, int b) {if(a>b)a=b;}

typedef pair<int, pair<int, int>> edge;

class UnionFind {
private:
	vector<int> par;
public:
	UnionFind(int N) { par = vector<int>(N, -1LL); }
	int find(int x);
	ll size(int x);
	void unite(int x, int y);
	bool same(int x, int y);
};



class Kruskal {
private:
	UnionFind *uf;
	vector<edge> e;
public:
	vector<edge> mst;
	Kruskal(int N) { uf = new UnionFind(N); }
	void add(int x, int y, ll z);
	void run();
};



//----UnionFind-------------------------------
int UnionFind::find(int x) {
	if (par[x] < 0) return x;
	else return par[x] = find(par[x]);
}

ll UnionFind::size(int x) {
	return -par[find(x)];
}

void UnionFind::unite(int x, int y) {
	x = find(x);
	y = find(y);

	//大きい方に小さい方をくっ付ける
	if (size(x) < size(y)) swap(x, y);
	par[x] += par[y];
	par[y] = x;
}

bool UnionFind::same(int x, int y) {
	x = find(x);
	y = find(y);
	return x == y;
}



//----Kruskal-------------------------------
void Kruskal::add(int x, int y, ll z) {
	//x < y
	if (x > y) swap(x, y);
	e.push_back({ z,{x,y} });
}

void Kruskal::run() {
	sort(e.begin(), e.end());
	e.erase(unique(e.begin(), e.end()), e.end());
	for (auto x : e) {
		if (uf->same(x.second.first, x.second.second)) {
			continue;
		}
		else {
			mst.push_back(x);
			uf->unite(x.second.first, x.second.second);
		}
	}
}

int hen[707][707];
int dp[707][707];
vector<int> col(707, -1);
int n;

pair<int, int> dfs(int u, int c) {
  pair<int, int> res = {0, 0};
  if (col[u] != -1) {
    if (c != col[u]) {
      cout << -1 << endl;
      exit(0);
    }
    return {0, 0};
  } else {
    col[u] = c;
    rep (to, n) {
      pair<int, int> tmp = {0, 0};
      if (hen[u][to]) tmp = dfs(to, c ^ 1);
      res.F += tmp.F, res.S += tmp.S;
    }
    if (c) res.S++; else res.F++;
    return res;
  }
}

signed main() {
  int m; cin >> n >> m;
  rep (i, 707) rep (j, 707) hen[i][j] = 1;
  rep(i, 707)hen[i][i]=0;
  rep (i, m) {
    int a, b; cin >> a >> b; --a, --b;
    hen[a][b] = hen[b][a] = 0;
  }
  vector<pair<int, int>> cand;
  rep (i, n) if (col[i] == -1) cand.pb(dfs(i, 0));
  int sum = 0;
  dp[0][0] = 1;
  for (auto elm : cand) {
    rep (i, sum + 1) if (dp[i][sum-i]) dp[i+elm.F][sum-i+elm.S] = dp[i+elm.S][sum-i+elm.F] = 1;
    sum += elm.F+elm.S;
  }
  int ans = INF;
  FOR (i, 1, n) {
    if (dp[i][sum-i]) {
      int a = i, b = sum - i;
      chmin(ans,a*(a-1)/2+b*(b-1)/2);
    }
  }
  cout << ans << endl;
}

Measured memory_bytes: 8448