// includes
#include <bits/stdc++.h>

// macros
#define ll long long int
#define pb emplace_back
#define mk make_pair
#define pq priority_queue
#define FOR(i, a, b) for(int i=(a);i<(b);++i)
#define rep(i, n) FOR(i, 0, n)
#define rrep(i, n) for(int i=((int)(n)-1);i>=0;i--)
#define irep(itr, st) for(auto itr = (st).begin(); itr != (st).end(); ++itr)
#define irrep(itr, st) for(auto itr = (st).rbegin(); itr != (st).rend(); ++itr)
#define vrep(v, i) for(int i = 0; i < (v).size(); i++)
#define all(x) (x).begin(),(x).end()
#define sz(x) ((int)(x).size())
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end())
#define FI first
#define SE second
#define dump(a, n) for(int i = 0; i < n; i++)cout << a[i] << "\n "[i + 1 != n];
#define dump2(a, n, m) for(int i = 0; i < n; i++)for(int j = 0; j < m; j++)cout << a[i][j] << "\n "[j + 1 != m];
#define bit(n) (1LL<<(n))
#define INT(n) int n; cin >> n;
#define LL(n) ll n; cin >> n;
#define DOUBLE(n) double n; cin >> n;
using namespace std;

//  types
typedef pair<int, int> P;
typedef pair<ll, int> Pl;
typedef pair<ll, ll> Pll;
typedef pair<double, double> Pd;
typedef complex<double> cd;
 
// constants
const int inf = 1e9;
const ll linf = 1LL << 50;
const double EPS = 1e-10;
const int mod = 1e9 + 7;
const int dx[4] = {-1, 0, 1, 0};
const int dy[4] = {0, -1, 0, 1};

// solve
template <class T>bool chmax(T &a, const T &b){if(a < b){a = b; return 1;} return 0;}
template <class T>bool chmin(T &a, const T &b){if(a > b){a = b; return 1;} return 0;}
template <typename T> istream &operator>>(istream &is, vector<T> &vec){for(auto &v: vec)is >> v; return is;}
template <typename T> ostream &operator<<(ostream &os, const vector<T>& vec){for(int i = 0; i < vec.size(); i++){ os << vec[i]; if(i + 1 != vec.size())os << " ";} return os;}
template <typename T> ostream &operator<<(ostream &os, const set<T>& st){for(auto itr = st.begin(); itr != st.end(); ++itr){ os << *itr; auto titr = itr; if(++titr != st.end())os << " ";} return os;}
template <typename T> ostream &operator<<(ostream &os, const unordered_set<T>& st){for(auto itr = st.begin(); itr != st.end(); ++itr){ os << *itr; auto titr = itr; if(++titr != st.end())os << " ";} return os;}
template <typename T> ostream &operator<<(ostream &os, const multiset<T>& st){for(auto itr = st.begin(); itr != st.end(); ++itr){ os << *itr; auto titr = itr; if(++titr != st.end())os << " ";} return os;}
template <typename T> ostream &operator<<(ostream &os, const unordered_multiset<T>& st){for(auto itr = st.begin(); itr != st.end(); ++itr){ os << *itr; auto titr = itr; if(++titr != st.end())os << " ";} return os;}
template <typename T1, typename T2> ostream &operator<<(ostream &os, const pair<T1, T2> &p){os << p.first << " " << p.second; return os;}
template <typename T1, typename T2> ostream &operator<<(ostream &os, const map<T1, T2> &mp){for(auto itr = mp.begin(); itr != mp.end(); ++itr){ os << itr->first << ":" << itr->second; auto titr = itr; if(++titr != mp.end())os << " "; } return os;}
template <typename T1, typename T2> ostream &operator<<(ostream &os, const unordered_map<T1, T2> &mp){for(auto itr = mp.begin(); itr != mp.end(); ++itr){ os << itr->first << ":" << itr->second; auto titr = itr; if(++titr != mp.end())os << " "; } return os;}

int l1 = 0, l2 = 0;
vector<int> v1, v2;

struct GraphI {
  int n;
  vector<int> color;
  vector<vector<int> > vec;
  GraphI(int ns) {
    n = ns;
    color.resize(n, 0);
    vec.resize(n);
  }
  void adde(int at, int to){
    vec[at].push_back(to);
  }
  bool is_bipartile(){
    for(int i = 0; i < n; i++){
      if(color[i] == 0){
        l1 = l2 = 0;
        if(!dfs(i, 1)){
          return false;
        }
        v1.pb(l1);
        v2.pb(l2);
      }
    }
    return true;
  }
  bool dfs(int v, int c){
    color[v] = c;
    if(c == 1)l1++;
    else l2++;
    for(int i: vec[v]){
      if(color[i] == c){
        return false;
      }
      if(color[i] == 0 && !dfs(i, -c))return false;
    }
    return true;
  }
};

bool e[701][701];
bool dp[701][701];

int main(int argc, char const* argv[])
{
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  INT(n); INT(m);
  rep(i, m){
    int u, v;
    cin >> u >> v, u--, v--;
    e[u][v] = true;
    e[v][u] = true;
  }
  GraphI graph(n);
  rep(i, n){
    rep(j, n){
      if(i != j && !e[i][j])graph.adde(i, j);
    }
  }
  if(!graph.is_bipartile()){
    cout << -1 << endl;
    return 0;
  }
  dp[0][0] = true;
  rep(i, sz(v1)){
    rep(j, n+1){
      if(j-v1[i]>=0)dp[i+1][j] |= dp[i][j-v1[i]];
      if(j-v2[i]>=0)dp[i+1][j] |= dp[i][j-v2[i]];
    }
  }
  int res = inf;
  rep(i, n+1){
    if(dp[sz(v1)][i]){
      res = min(res, i * (i - 1) / 2 + (n - i) * (n - i - 1) / 2);
    }
  }
  cout << res << endl;
  return 0;
}


Measured memory_bytes: 3072