#include <bits/stdc++.h>
#define LLI long long int
#define FOR(v, a, b) for(LLI v = (a); v < (b); ++v)
#define FORE(v, a, b) for(LLI 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(LLI v = (a); v >= (b); --v)
#define ALL(x) (x).begin(), (x).end()
#define RALL(x) (x).rbegin(), (x).rend()
#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 fst first
#define snd second
#define popcount __builtin_popcount
#define UNIQ(v) (v).erase(unique(ALL(v)), (v).end())
#define bit(i) (1LL<<(i))

#ifdef DEBUG
#include <misc/C++/Debug.cpp>
#else
#define dump(...) ((void)0)
#endif

#define gcd __gcd

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

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

struct Init{
  Init(){
    cin.tie(0);
    ios::sync_with_stdio(false);
  }
}init;


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{
  vector<bool> is_bipartite_graph;
  vector<pair<int,int>> component_size;
  bool all_bipartite;
  
  CheckBipartiteGraph(const Graph<T> &g): all_bipartite(true){
    int n = g.size();

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

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

      int a,b;
      bool t = aux(g,i,check,visit,a,b);
      is_bipartite_graph.push_back(t);
      component_size.push_back(make_pair(a,b));

      if(not t){
        all_bipartite = false;
      }
    }
  }
  
  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);
    }

    auto res = CheckBipartiteGraph<int>(cogra);

    if(not res.all_bipartite){
      cout << -1 << endl;
    }else{
      vector<bool> dp(2*N+1);
      auto ps = res.component_size;

      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