#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);}

template <typename Cost = int> class Edge{
public:
  int from,to;
  Cost cost;
  Edge() {}
  Edge(int to, Cost cost): to(to), cost(cost){}
  Edge(int from, int to, Cost cost): from(from), to(to), cost(cost){}

  Edge rev() const {return Edge(to,from,cost);}
  
  friend ostream& operator<<(ostream &os, const Edge &e){
    os << "(FROM: " << e.from << "," << "TO: " << e.to << "," << "COST: " << e.cost << ")";
    return os;
  }
};

template <typename T> using Graph = vector<vector<Edge<T>>>;
template <typename T> using Tree = vector<vector<Edge<T>>>;

template <typename C, typename T> void add_edge(C &g, int from, int to, T w){
  g[from].push_back(Edge<T>(from, to, w));  
}

template <typename C, typename T> void add_undirected(C &g, int a, int b, T w){
  g[a].push_back(Edge<T>(a, b, w));
  g[b].push_back(Edge<T>(b, a, w));
}


template <typename T> struct CheckBipartiteGraph{

  static bool is_bipartite_graph(const Graph<T> &g, vector<pair<int,int>> &ret){
    int n = g.size();

    vector<int> check(n,-1);
    vector<bool> visit(n,false);

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

      int a,b;
      if(aux(g,i,check,visit,a,b)){
        ret.push_back(make_pair(a,b));
      }else{
        return false;
      }
    }
    
    return true;
  }
  
  static bool aux(const Graph<T> &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 &e : graph[cur]){
        if(check[e.to] == check[cur]) return false;
 
        if(check[e.to] == -1){
          if(check[cur] == 0){
            check[e.to] = 1;
            ++b;
          }else{
            check[e.to] = 0;
            ++a;
          }
          st.push(e.to);
        }
      }
    }
  
    return true;
  }
};


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

  int N,M;
  while(cin >> N >> M){
    Graph<int> graph(N);
    REP(i,M){
      int a,b; cin >> a >> b; --a, --b;
      add_undirected(graph,a,b,1);
    }

    Graph<int> cogra(N);

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

      REP(j,N) if(t[j]) add_edge(cogra,i,j,1);
    }

    vector<pair<int,int>> ps;
    bool no_ans = !CheckBipartiteGraph<int>::is_bipartite_graph(cogra,ps);

    if(no_ans){
      cout << -1 << endl;
    }else{
      vector<bool> dp(2*N+1);

      REP(i,(int)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: 9472