//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2,avx512f")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include <immintrin.h>
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cmath>
#include <complex>
#include <functional>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <memory>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <vector>
#ifdef DEBUG
#include "./CompetitiveProgrammingCpp/Timer.hpp"
#include "./CompetitiveProgrammingCpp/debug.hpp"
#include "./CompetitiveProgrammingCpp/sample.hpp"
#else
#define dump(...)
#endif

/* macro */
#define FOR(i, b, e) for (ll i = (ll)(b); i < (ll)(e); ++i)
#define RFOR(i, b, e) for (ll i = (ll)(e - 1); i >= (ll)(b); --i)
#define REP(i, n) FOR(i, 0, n)
#define RREP(i, n) RFOR(i, 0, n)
#define REPC(x, c) for (const auto& x : (c))
#define REPI2(it, b, e) for (auto it = (b); it != (e); ++it)
#define REPI(it, c) REPI2(it, (c).begin(), (c).end())
#define RREPI(it, c) REPI2(it, (c).rbegin(), (c).rend())
#define REPI_ERACE2(it, b, e) for (auto it = (b); it != (e);)
#define REPI_ERACE(it, c) REPI_ERACE2(it, (c).begin(), (c).end())
#define ALL(x) (x).begin(), (x).end()
#define cauto const auto&
/* macro func */
template <class T>
inline auto sort(T& t) {
  std::sort(ALL(t));
}
template <class T>
inline auto rsort(T& t) {
  std::sort((t).rbegin(), (t).rend());
}
template <class T>
inline auto unique(T& t) {
  (t).erase(unique((t).begin(), (t).end()), (t).end());
}
template <class T, class S>
inline auto chmax(T& t, const S& s) {
  if (s > t) {
    t = s;
    return true;
  }
  return false;
}
template <class T, class S>
inline auto chmin(T& t, const S& s) {
  if (s < t) {
    t = s;
    return true;
  }
  return false;
}
inline auto BR() { std::cout << "\n"; }

/* type define */
using ll = long long;
using PAIR = std::pair<ll, ll>;
using VS = std::vector<std::string>;
using VL = std::vector<long long>;
using VVL = std::vector<VL>;
using VVVL = std::vector<VVL>;
using VD = std::vector<double>;
template <class T>
using V = std::vector<T>;

/* using std */
using std::cout;
constexpr char endl = '\n';
using std::bitset;
using std::cin;
using std::list;
using std::map;
using std::multimap;
using std::multiset;
using std::pair;
using std::priority_queue;
using std::queue;
using std::set;
using std::stack;
using std::string;
using std::unordered_map;
using std::unordered_multimap;
using std::unordered_multiset;
using std::unordered_set;
using std::vector;

/* Initial processing  */
struct Preprocessing {
  Preprocessing() {
    std::cin.tie(0);
    std::ios::sync_with_stdio(0);
  };
} _Preprocessing;

/* Remove the source of the bug */
signed pow(signed, signed) {
  assert(false);
  return -1;
}

/* define hash */
namespace std {
template <>
class hash<std::pair<ll, ll>> {
 public:
  size_t operator()(const std::pair<ll, ll>& x) const {
    return hash<ll>()(1000000000 * x.first + x.second);
  }
};
}  // namespace std

/* input */
template <class T>
std::istream& operator>>(std::istream& is, vector<T>& vec) {
  for (T& x : vec) is >> x;
  return is;
}

/* constant value */
constexpr ll MOD = 1000000007;
// constexpr ll MOD = 998244353;

//=============================================================================================

signed main() {
  ll n, m;
  cin >> n >> m;
  unordered_set<PAIR> st;
  REP(_, m) {
    ll a, b;
    cin >> a >> b;
    if (a > b) {
      std::swap(a, b);
    }
    --a;
    --b;
    st.emplace(a, b);
  }
  unordered_multimap<ll, ll> graph;
  REP(i, n) FOR(j, i + 1, n) if (st.find(PAIR{i, j}) == st.end()) {
    graph.emplace(i, j);
    graph.emplace(j, i);
  }

  VL isUsed(n, -1);
  V<PAIR> es;
  es.reserve(n);
  REP(f, n) if (isUsed[f] == -1) {
    isUsed[f] = 0;
    PAIR p{0, 0};

    queue<PAIR> q;
    q.emplace(f, 0);
    while (!q.empty()) {
      auto [from, b] = q.front();
      q.pop();
      ++(b ? p.first : p.second);
      ll k = (b + 1) & 1;
      auto range = graph.equal_range(from);
      REPI2(itr, range.first, range.second) {
	ll to = itr->second;
	if (isUsed[to] == -1) {
	  isUsed[to] = k;
	  q.emplace(to, k);
	} else {
	  if (isUsed[to] != k) {
	    cout << -1 << endl;
	    return 0;
	  }
	}
      }
    }
    es.emplace_back(p);
  }
  constexpr ll mx = 1e6;
  bitset<mx> dp = 1;
  for (auto [a, b] : es) {
    dp = (dp << a) | (dp << b);
  }
  ll ans = mx;
  ll tmp = n * (n - 1) / 2;
  REP(i, mx) if (dp[i]) { chmin(ans, tmp - (n - i) * i); }
  cout << ans << endl;
}


Measured memory_bytes: 22512