#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <iostream>
#include <istream>
#include <iterator>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>
#include <tuple>
#include <iomanip>
#include <climits>
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
//typedef pair<ll, ll> P;
#define rep(i, n) for(ll i = 0; i < (n); i++)
#define revrep(i, n) for(ll i = (n)-1; i >= 0; i--)
#define pb push_back
#define f first
#define s second
#define chmin(x, y) x = min(x, y);
#define chmax(x, y) x = max(x, y);

//const ll INFL = LLONG_MAX;//10^18 = 2^60
const ll INFL = 1LL<<60;
//const int INF = INT_MAX;
const ll INF = 1 << 30;//10^9
ll MOD = 1000000007;
//ll MOD = 998244353;

vector<ll> dy = {0, 1, 0, -1, 1, 1, -1, -1, 0};
vector<ll> dx = {1, 0, -1, 0, 1, -1, 1, -1, 0};

void pres(double A, ll x = 10){cout << fixed << setprecision(x) << A << endl;}
void BinarySay(ll x, ll y = 60){rep(i, y) cout << (x>>(y-1-i) & 1); cout << endl;}
ll cnt_bit(ll x){return __builtin_popcountll(x);}

ll pow_long(ll x, ll k){
  ll res = 1;
  while(k > 0){
    if(k % 2) res *= x;
    x *= x; k /= 2;
  }
  return res;
}

ll pow_mod(ll x, ll k){
  ll res = 1;
  while(k > 0){
    if(k % 2){
      res *= x; res %= MOD;
    }
    x *= x; x %= MOD;
    k /= 2;
  }
  return res;
}

ll inverse(ll x){return pow_mod(x, MOD - 2);};

ll gcd(ll a, ll b){
    if(b == 0) return a;
    return gcd(b, a % b);
}

ll lcm(ll x, ll y){
  ll res = x / gcd(x, y);
  res *= y;
  return res;
};

//コンビネーション
const int MAXcomb = 200010;
ll fac[MAXcomb], finv[MAXcomb], inv[MAXcomb];
//facはn!,finvは1/n!
//invは逆元
void COMinit(){
    fac[0] = fac[1] = 1;
    finv[0] = finv[1] = 1;
    inv[1] = 1;
    for(int i = 2; i < MAXcomb; i++){
        fac[i] = fac[i-1] * i % MOD;
        inv[i] = MOD - inv[MOD%i] * (MOD/i) % MOD;
        finv[i] = finv[i-1] * inv[i] % MOD;
    }
}
ll comb(int n, int k){
    if(n < k) return 0;
    if(n < 0 || k < 0) return 0;
    return fac[n] * finv[k] % MOD * finv[n-k] % MOD;
}

const int MAXkai = 200010;
ll kai_memo[MAXkai];
ll kai(ll N){
  if(kai_memo[N] != 0) return kai_memo[N];
  if(N <= 1) return 1;
  return kai_memo[N] = N * kai(N-1) % MOD;
}

ll N, M;
vector<ll> E[710];
vector<ll> rE[710];
ll visited[710];
bool boolBinary = 1;
set<ll> o, x;

void make_binary(ll now, ll col){
  visited[now] = col;
  if(col == 1) o.insert(now);
  else x.insert(now);
  for(ll to: rE[now]){
    if(visited[to] == 0) make_binary(to, -col);
    else if(visited[to] == col){
      boolBinary = 0;
      return;
    }
  }
}

void solve(){
  cin >> N >> M;
  rep(i, M){
    ll a, b;
    cin >> a >> b;
    a--, b--;
    E[a].pb(b);
    E[b].pb(a);
  }
  rep(i, N){
    set<ll> S;
    rep(j, N)if(j != i) S.insert(j);
    for(ll to: E[i]) S.erase(to);
    for(auto itr = S.begin(); itr != S.end(); itr++){
      rE[i].pb(*itr);
    }
  }
  vector<pair<ll, ll>> B;
  rep(i, N){
    if(visited[i] != 0) continue;
    make_binary(i, 1);
    if(boolBinary == 0){
      cout << -1 << endl;
      return;
    }
    B.pb({o.size(), x.size()});
    o.clear();
    x.clear();
  }

  M = B.size();
  bitset<1000> bset;
  bset[0] = 1;
  rep(i, M){
    bset = (bset << B[i].f) | (bset << B[i].s);
  }
  ll ans = INFL;
  rep(i, 1000){
    if(bset[i]){
      ans = min(ans, i * (i - 1) / 2 + (N - i) * (N - i - 1) / 2);
    }
  }
  cout << ans << endl;
}


int main(){
  cin.tie(0);
  ios::sync_with_stdio(false);
  solve();
}


Measured memory_bytes: 6400