#include <bits/stdc++.h>

#define rep(i, n) for (int i = 0; i < int(n); i++)
#define rrep(i, n) for (int i = int(n) - 1; i >= 0; i--)
#define reps(i, n) for (int i = 1; i <= int(n); i++)
#define rreps(i, n) for (int i = int(n); i >= 1; i--)
#define repc(i, n) for (int i = 0; i <= int(n); i++)
#define rrepc(i, n) for (int i = int(n); i >= 0; i--)
#define repi(i, a, b) for (int i = int(a); i < int(b); i++)
#define repic(i, a, b) for (int i = int(a); i <= int(b); i++)
#define all(a) (a).begin(), (a).end()
#define bit32(x) (1 << (x))
#define bit64(x) (1ll << (x))
#define sz(v) ((int) v.size())

using namespace std;

using i64 = long long;
using f80 = long double;
using vi32 = vector<int>;
using vi64 = vector<i64>;
using vf80 = vector<f80>;
using vstr = vector<string>;

void yes() { cout << "Yes" << endl; exit(0); }
void no() { cout << "No" << endl; exit(0); }
template <typename T> class pqasc : public priority_queue<T, vector<T>, greater<T>> {};
template <typename T> class pqdesc : public priority_queue<T, vector<T>, less<T>> {};
template <typename T> void amax(T &x, T y) { x = max(x, y); }
template <typename T> void amin(T &x, T y) { x = min(x, y); }
template <typename T> T exp(T x, i64 n, T e = 1) { T r = e; while (n > 0) { if (n & 1) r *= x; x *= x; n >>= 1; } return r; }
template <typename T> istream& operator>>(istream &is, vector<T> &v) { for (auto &x : v) is >> x; return is; }
template <typename T> ostream& operator<<(ostream &os, vector<T> &v) { rep(i, v.size()) { if (i) os << ' '; os << v[i]; } return os; }
void solve(); int main() { ios::sync_with_stdio(0); cin.tie(0); cout << fixed << setprecision(16); solve(); return 0; }

// http://koturn.hatenablog.com/entry/2018/08/01/010000
template <typename T, typename U>
vector<U> make_vector(T&& n, U&& val) {
  return vector<U>(forward<T>(n), forward<U>(val));
}
template <typename T, typename... Args>
decltype(auto) make_vector(T&& n, Args&&... args) {
  return vector<decltype(make_vector(forward<Args>(args)...))>(forward<T>(n), make_vector(forward<Args>(args)...));
}

const int INF = 1001001001;
const int dx[] = {1, 0, -1, 0};
const int dy[] = {0, 1, 0, -1};

void solve() {
  int n, m;
  cin >> n >> m;
  auto g = make_vector(n, n, 0);
  rep(i, m) {
    int a, b;
    cin >> a >> b;
    a--, b--;
    g[a][b] = 1;
    g[b][a] = 1;
  }
  vector<vi32> edges(n);
  rep(i, n) rep(j, n) if (i != j) {
    g[i][j] = 1 - g[i][j];
    if (g[i][j] == 1) {
      edges[i].push_back(j);
    }
  }
  vi32 dist(n, INF);
  int even, odd;
  auto dfs = [&](auto dfs, int u, int d) -> bool {
    if (dist[u] != INF) {
      return dist[u] % 2 == d % 2;
    }
    dist[u] = d;
    if (d % 2 == 0) {
      even += 1;
    } else {
      odd += 1;
    }
    for (auto v : edges[u]) {
      if (!dfs(dfs, v, d + 1)) {
        return false;
      }
    }
    return true;
  };
  using P = pair<int, int>;
  vector<P> eos;
  rep(i, n) if (dist[i] == INF) {
    even = 0, odd = 0;
    bool res = dfs(dfs, i, 0);
    if (!res) {
      cout << -1 << endl;
      return;
    }
    eos.emplace_back(even, odd);
  }
  auto dp = make_vector(sz(eos) + 1, 701, false);
  {
    int n = sz(eos);
    dp[0][0] = true;
    rep(i, n) {
      vi32 v(2);
      v[0] = eos[i].first;
      v[1] = eos[i].second;
      repc(j, 700) rep(k, 2) {
        if (!dp[i][j]) continue;
        dp[i + 1][j + v[k]] = true;
      }
    }
  }
  int ans = INF;
  repc(j, n) if (dp[sz(eos)][j]) {
    int a = j;
    int b = n - j;
    amin(ans, a * (a - 1) / 2 + b * (b - 1) / 2);
  }
  cout << ans << endl;
}


Measured memory_bytes: 7940