#include <iostream>
#include <vector>
#include <cstdio>
#include <sstream>
#include <map>
#include <string>
#include <algorithm>
#include <queue>
#include <cmath>
#include <functional>
#include <set>
#include <ctime>
#include <random>
#include <chrono>
#include <cassert>
#include <tuple>
#include <utility>
using namespace std;

namespace {
  using Integer = long long; //__int128;
  template<class T, class S> istream& operator >> (istream& is, pair<T,S>& p){return is >> p.first >> p.second;}
  template<class T> istream& operator >> (istream& is, vector<T>& vec){for(T& val: vec) is >> val; return is;}
  template<class T> istream& operator ,  (istream& is, T& val){ return is >> val;}
  template<class T, class S> ostream& operator << (ostream& os, const pair<T,S>& p){return os << p.first << " " << p.second;}
  template<class T> ostream& operator << (ostream& os, const vector<T>& vec){for(size_t i=0; i<vec.size(); i++) os << vec[i] << (i==vec.size()-1?"":" "); return os;}
  template<class T> ostream& operator ,  (ostream& os, const T& val){ return os << " " << val;}

  template<class H> void print(const H& head){ cout << head; }
  template<class H, class ... T> void print(const H& head, const T& ... tail){ cout << head << " "; print(tail...); }
  template<class ... T> void println(const T& ... values){ print(values...); cout << endl; }

  template<class H> void eprint(const H& head){ cerr << head; }
  template<class H, class ... T> void eprint(const H& head, const T& ... tail){ cerr << head << " "; eprint(tail...); }
  template<class ... T> void eprintln(const T& ... values){ eprint(values...); cerr << endl; }

  class range{ Integer start_, end_, step_; public: struct range_iterator{ Integer val, step_; range_iterator(Integer v, Integer step) : val(v), step_(step) {} Integer operator * (){return val;} void operator ++ (){val += step_;} bool operator != (range_iterator& x){return step_ > 0 ? val < x.val : val > x.val;} }; range(Integer len) : start_(0), end_(len), step_(1) {} range(Integer start, Integer end) : start_(start), end_(end), step_(1) {} range(Integer start, Integer end, Integer step) : start_(start), end_(end), step_(step) {} range_iterator begin(){ return range_iterator(start_, step_); } range_iterator   end(){ return range_iterator(  end_, step_); } };

  inline string operator "" _s (const char* str, size_t size){ return move(string(str)); }
  constexpr Integer my_pow(Integer x, Integer k, Integer z=1){return k==0 ? z : k==1 ? z*x : (k&1) ? my_pow(x*x,k>>1,z*x) : my_pow(x*x,k>>1,z);}
  constexpr Integer my_pow_mod(Integer x, Integer k, Integer M, Integer z=1){return k==0 ? z%M : k==1 ? z*x%M : (k&1) ? my_pow_mod(x*x%M,k>>1,M,z*x%M) : my_pow_mod(x*x%M,k>>1,M,z);}
  constexpr unsigned long long operator "" _ten (unsigned long long value){ return my_pow(10,value); }

  inline int k_bit(Integer x, int k){return (x>>k)&1;} //0-indexed

  mt19937 mt(chrono::duration_cast<chrono::nanoseconds>(chrono::steady_clock::now().time_since_epoch()).count());

  template<class T> string join(const vector<T>& v, const string& sep){ stringstream ss; for(size_t i=0; i<v.size(); i++){ if(i>0) ss << sep; ss << v[i]; } return ss.str(); }

  inline string operator * (string s, int k){ string ret; while(k){ if(k&1) ret += s; s += s; k >>= 1; } return ret; }
}
constexpr long long mod = 9_ten + 7;

int x,y;

bool dfs(vector<vector<int>>& G, vector<int>& c, int pos, int color){
  c[pos] = color;
  if(color==0) x++;
  else y++;

  for(auto nx : G[pos]){
    if( c[nx] != -1 && c[nx] != (color^1) ) return false;
    if( c[nx] != -1 ) continue;
    bool ok = dfs(G, c, nx, color^1);
    if(!ok) return false;
  }
  return true;
}

int main(){
  int n,m;
  cin >> n,m;

  vector<vector<int>> E(n, vector<int>(n, 0));
  for(int i=0; i<m; i++){
    int a,b;
    cin >> a,b;
    a--; b--;
    E[a][b] = E[b][a] = 1;
  }

  vector<vector<int>> F(n);
  for(int i=0; i<n; i++){
    for(int j=0; j<n; j++){
      if(i==j) continue;
      if( E[i][j] == 0 ) F[i].push_back(j);
    }
  }


  vector<int> dp(n+1, 0);
  dp[0] = 1;

  vector<int> c(n, -1);
  for(int i=0; i<n; i++){
    if( c[i] != -1 ) continue;
    x=0, y=0;

    bool ok = dfs(F, c, i, 0);
    if(!ok){
      println("-1");
      return 0;
    }

    vector<int> dp_(n+1, 0);
    for(int i=0; i<=n; i++){
      if(dp[i] == 0) continue;
      dp_[i+x] = dp[i];
      dp_[i+y] = dp[i];
    }
    dp = dp_;
  }

  long long ans = 1ll<<50;
  for(int i=0; i<=n; i++){
    if( dp[i] == 0 ) continue;
    long long e = i*(i-1)/2 + (n-i)*(n-i-1)/2;
    ans = min(ans, e);
  }

  println(ans);




  return 0;
}


Measured memory_bytes: 4992