#define STOPIT
#include <bits/stdc++.h>
#define loop(n) for (lint ngtkana_is_a_genius = 0; ngtkana_is_a_genius < lint(n); ngtkana_is_a_genius++)
#define rep(i, begin, end) for (lint i = lint(begin); (i) < lint(end); i++)
#define all(v) v.begin(), v.end()
#define rand(l, r) std::uniform_int_distribution<>(l, r)(mt)
using lint = long long;
auto mt = std::mt19937_64(std::random_device{}());
auto cmn = [](auto&& a, auto b){ if (a > b) {a = b; return true;} return false; };
auto cmx = [](auto&& a, auto b){ if (a < b) {a = b; return true;} return false; };
void debug_impl() { std::cerr << std::endl; }
template <typename Head, typename... Tail>
void debug_impl(Head head, Tail... tail) { std::cerr << " " << head; debug_impl(tail...); }
#ifndef STOPIT
#define debug(...)\
  do {\
    std::cerr << std::boolalpha << "[" << #__VA_ARGS__ << "]:";\
    debug_impl(__VA_ARGS__);\
    std::cerr << std::noboolalpha;\
  } while (false)
#else
#define debug(...) {}
#endif

template < typename Container, typename Value = typename Container::value_type, std::enable_if_t<!std::is_same< Container, std::string >::value, std::nullptr_t> = nullptr>
std::istream& operator>> (std::istream& is, Container& v)
  { for (auto & x : v) { is >> x; } return is; }

template < typename Container, typename Value = typename Container::value_type, std::enable_if_t<!std::is_same< Container, std::string >::value, std::nullptr_t> = nullptr >
std::ostream& operator<< (std::ostream& os, Container const& v) {
 os << "{";
  for (auto it = v.begin(); it != v.end(); it++)
    {os << (it != v.begin() ? "," : "") << *it;}
  return os << "}";
}

template <typename T>
auto make_vector_impl(size_t sz, T t) {return std::vector<T>(sz, t);}

template <size_t N, typename T, typename U, std::enable_if_t<N == 1, std::nullptr_t> = nullptr>
auto make_vector(size_t sz, U u) {return make_vector_impl(sz, T(u));}

template <size_t N, typename T, std::enable_if_t<N == 1, std::nullptr_t> = nullptr>
auto make_vector(size_t sz) {return std::vector<T>(sz);}

template <size_t N, typename T, typename... Args, std::enable_if_t<N != 1, std::nullptr_t> = nullptr>
auto make_vector(size_t a, Args... args) {return make_vector_impl(a, make_vector<N - 1, T>(args...));}

template <typename T, typename Size_t>
auto& at(T& t, Size_t i) {return t.at(i);}

template <typename T, typename Size_t, typename... Args>
auto& at(T& t, Size_t i, Args... args) {return at(t.at(i), args...);}

void err() { std::cout << -1 << std::endl; exit(0); }
#define require(exp) do { if (!(exp)) { err(); } } while(false)

int main() {
  std::cin.tie(0); std::cin.sync_with_stdio(false);
  int n, m; std::cin >> n >> m;
  auto adj = make_vector<2,int>(n,n,0);
  loop(m) {
    int u, v; std::cin >> u >> v;
    u--,v--;
    if (u > v) std::swap(u,v);
    at(adj,u,v) = 1;
  }
  std::vector<std::vector<int>> graph(n);
  rep(i,0,n) rep(j,i+1,n) {
    if (at(adj,i,j)) continue;
    graph.at(i).emplace_back(j);
    graph.at(j).emplace_back(i);
  }
  debug(graph);

  std::vector<int> par(n,-1);
  auto dfs = [&] (auto&& dfs, int x, int px, std::vector<int>& sz) -> void {
    sz.at(px)++;
    debug(x,sz);
    for (int y : graph.at(x)) {
      int& py = par.at(y);
      if (py != -1) {
        require(px!=py);
        continue;
      }
      py = !px;
      dfs(dfs,y,py,sz);
    }
  };
  std::vector<int> dp(n+1,false);
  dp.at(0) = true;
  rep(i,0,n) {
    if (par.at(i)!=-1)continue;
    std::vector<int> sz(2);
    par.at(i) = 0;
    dfs(dfs,i,0,sz);
    debug(sz);
    for (int i = n; i >= 0; i--) {
      for (int x : sz) {
        if (i+x<=n) {
          dp.at(i+x) |= dp.at(i);
        }
      }
      if (sz.at(0)!=0 && sz.at(1)!=0) {
        dp.at(i) = false;
      }
    }
  }
  debug(par);
  debug(dp);

  auto tri = [&] (lint k) {
    return k * (k - 1) / 2;
  };
  lint ans = tri(n);
  rep(i,0,n+1) {
    if (!dp.at(i)) continue;
    lint now = tri(i) + tri(n-i);
    debug(now);
    cmn(ans,now);
  }
  std::cout << ans << std::endl;
  return 0;
}

/*
最も均等なグループ分けをすればよいです。
このグループ分けは、補グラフが二部グラフならばよいです。
ナップザックです。
*/

Measured memory_bytes: 5120