#include <bits/stdc++.h>
#define FOR(v, a, b) for(int v = (a); v < (b); ++v)
#define FORE(v, a, b) for(int v = (a); v <= (b); ++v)
#define REP(v, n) FOR(v, 0, n)
#define REPE(v, n) FORE(v, 0, n)
#define REV(v, a, b) for(int v = (a); v >= (b); --v)
#define ALL(x) (x).begin(), (x).end()
#define ITR(it, c) for(auto it = (c).begin(); it != (c).end(); ++it)
#define RITR(it, c) for(auto it = (c).rbegin(); it != (c).rend(); ++it)
#define EXIST(c,x) ((c).find(x) != (c).end())
#define LLI long long int
#define fst first
#define snd second
#ifndef M_PI
#define M_PI 3.14159265358979323846
#endif
#ifndef M_E
#define M_E 2.71828182845904523536
#endif
#ifdef DEBUG
#include <boost/core/demangle.hpp>
#define dump(x) cerr << "L" << __LINE__ << ": in " << __PRETTY_FUNCTION__ << " \e[32;1m" << boost::core::demangle(typeid(x).name()) << "\e[37m" << " " << (#x) << " = " << (x) << "\e[m" << endl;
#else
#define dump(x)
#endif
#define pln(x) cout << (x) << endl
#define gcd __gcd

using namespace std;
template <class T> constexpr T lcm(T m, T n){return m/gcd(m,n)*n;}

template <typename T> using V = vector<T>;
template <typename T, typename U> using P = pair<T,U>;
template <typename I> void join(ostream &ost, I s, I t, string d=" "){for(auto i=s; i!=t; ++i){if(i!=s)ost<<d; ost<<*i;}ost<<endl;}
template <typename T> istream& operator>>(istream &is, vector<T> &v){for(auto &a : v) is >> a; return is;}
template <typename T, typename U> istream& operator>>(istream &is, pair<T,U> &p){is >> p.first >> p.second; return is;}

template <typename Iter> ostream& out_container(ostream &os, Iter first, Iter last){
  os << "{"; auto itr = first; while(itr != last){if(itr != first) os << ","; os << *itr; ++itr;} os << "}"; return os;
}

template <typename T> ostream& operator<<(ostream &os, const vector<T> &c){return out_container(os,ALL(c));}
template <typename T> ostream& operator<<(ostream &os, const deque<T> &c){return out_container(os,ALL(c));}
template <typename T> ostream& operator<<(ostream &os, const set<T> &c){return out_container(os,ALL(c));}
template <typename T> ostream& operator<<(ostream &os, const unordered_set<T> &c){return out_container(os,ALL(c));}
template <typename T, typename U> ostream& operator<<(ostream &os, const map<T,U> &c){return out_container(os,ALL(c));}
template <typename T, typename U> ostream& operator<<(ostream &os, const unordered_map<T,U> &c){return out_container(os,ALL(c));}
template <typename T, typename U> ostream& operator<<(ostream& os, const pair<T,U> &p){os << "(" << p.first << "," << p.second << ")"; return os;}

template <typename T, typename U> T& chmin(T &a, const U &b){return a = (a<=b?a:b);}
template <typename T, typename U> T& chmax(T &a, const U &b){return a = (a>=b?a:b);}
template <typename T, size_t N, typename U> void fill_array(T (&a)[N], const U &v){fill((U*)a, (U*)(a+N), v);}


bool is_bipartite_graph(vector<vector<int>> &graph, int root, vector<int> &check, vector<bool> &visit, int &a, int &b){
  stack<int> st;
 
  a = 1;
  b = 0;
  st.push(root);
  check[root] = 0;
 
  while(!st.empty()){
    auto cur = st.top(); st.pop();
    if(visit[cur]) continue;
    visit[cur] = true;
 
    for(auto next : graph[cur]){
      if(check[next] == check[cur]) return false;
 
      if(check[next] == -1){
	if(check[cur] == 0){
	  check[next] = 1;
	  ++b;
	}else{
	  check[next] = 0;
	  ++a;
	}
	st.push(next);
      }
    }
  }
  
  return true;
}


int main(){
  cin.tie(0);
  ios::sync_with_stdio(false);

  int N,M;
  while(cin >> N >> M){
    vector<vector<int>> graph(N);
    REP(i,M){
      int a,b; cin >> a >> b; --a, --b;
      graph[a].push_back(b);
      graph[b].push_back(a);
    }

    vector<vector<int>> cogra(N);

    REP(i,N){
      vector<int> t(N,1);
      t[i] = 0;
      for(auto x : graph[i]) t[x] = 0;

      REP(j,N) if(t[j]) cogra[i].push_back(j);
    }

    vector<bool> visit(N);
    vector<int> check(N,-1);
    vector<pair<int,int>> ps;
    bool no_ans = false;

    
    REP(i,N){
      if(visit[i]) continue;

      int a,b;
      if(!is_bipartite_graph(cogra, i, check, visit, a, b)){
	no_ans = true;
	break;
      }else{
	ps.push_back({a,b});
      }
    }
    
    if(no_ans){
      cout << -1 << endl;
    }else{


      vector<bool> dp(2*N+1);

      REP(i,ps.size()){
	auto &p = ps[i];
	vector<bool> dp1(2*N+1);
	if(!i) dp1[p.fst] = true;
	REPE(i,N) dp1[i+p.fst] = dp1[i+p.fst] || dp[i];

	vector<bool> dp2(2*N+1);
	if(!i) dp2[p.snd] = true;
	REPE(i,N) dp2[i+p.snd] = dp2[i+p.snd] || dp[i];

	REPE(i,N) dp[i] = dp1[i] || dp2[i];
      }

      int ans = 1e9;
      REPE(i,N){
	if(dp[i]) chmin(ans, i*(i-1)/2 + (N-i)*(N-i-1)/2);
      }

      cout << ans << endl;
    }
  }
  
  return 0;
}


Measured memory_bytes: 3328