#include<bits/stdc++.h>
using namespace std;

#define endl '\n'
#define lfs cout<<fixed<<setprecision(10)
#define ALL(a)  (a).begin(),(a).end()
#define ALLR(a)  (a).rbegin(),(a).rend()
#define spa << " " <<
#define fi first
#define se second
#define MP make_pair
#define MT make_tuple
#define PB push_back
#define EB emplace_back
#define rep(i,n,m) for(ll i = (n); i < (ll)(m); i++)
#define rrep(i,n,m) for(ll i = (m) - 1; i >= (ll)(n); i--)
using ll = long long;
using ld = long double;
const ll MOD = 1e9+7;
//const ll MOD = 998244353;
const ll INF = 1e18;
using P = pair<ll, ll>;
template<typename T>
bool chmin(T &a,T b){if(a>b){a=b;return true;}else return false;}
template<typename T>
bool chmax(T &a,T b){if(a<b){a=b;return true;}else return false;}
ll median(ll a,ll b, ll c){return a+b+c-max({a,b,c})-min({a,b,c});}
void ans1(bool x){if(x) cout<<"Yes"<<endl;else cout<<"No"<<endl;}
void ans2(bool x){if(x) cout<<"YES"<<endl;else cout<<"NO"<<endl;}
void ans3(bool x){if(x) cout<<"Yay!"<<endl;else cout<<":("<<endl;}
template<typename T1,typename T2>
void ans(bool x,T1 y,T2 z){if(x)cout<<y<<endl;else cout<<z<<endl;}  
template<typename T>
void debug(vector<vector<T>>&v,ll h,ll w){for(ll i=0;i<h;i++)
{cout<<v[i][0];for(ll j=1;j<w;j++)cout spa v[i][j];cout<<endl;}};
void debug(vector<string>&v,ll h,ll w){for(ll i=0;i<h;i++)
{for(ll j=0;j<w;j++)cout<<v[i][j];cout<<endl;}};
template<typename T>
void debug(vector<T>&v,ll n){if(n!=0)cout<<v[0];
for(ll i=1;i<n;i++)cout spa v[i];cout<<endl;};
template<typename T>
vector<vector<T>>vec(ll x, ll y, T w){
  vector<vector<T>>v(x,vector<T>(y,w));return v;}
ll gcd(ll x,ll y){ll r;while(y!=0&&(r=x%y)!=0){x=y;y=r;}return y==0?x:y;}
vector<ll>dx={1,0,-1,0,1,1,-1,-1};
vector<ll>dy={0,1,0,-1,1,-1,1,-1};
template<typename T>
vector<T> make_v(size_t a,T b){return vector<T>(a,b);}
template<typename... Ts>
auto make_v(size_t a,Ts... ts){
  return vector<decltype(make_v(ts...))>(a,make_v(ts...));
}
ostream &operator<<(ostream &os, pair<ll, ll>&p){
  return os << p.first << " " << p.second;
}  
bool judgebuf=true;
vector<ll>used(1000,-1);
ll n,m;
void dfs(ll k,vector<ll>&cnt,vector<vector<bool>>&g,ll d){
  used[k]=d;
  cnt[d]++;
  rep(i,0,n){
    if(!g[k][i])continue;
    if(used[i]==d)judgebuf=false;
    if(used[i]==-1)dfs(i,cnt,g,d^1);
  }
}
int main(){
  cin.tie(nullptr);
  ios_base::sync_with_stdio(false);
  ll res=0,buf=0;
  bool judge = true;
  cin>>n>>m;
  auto g=vec(n,n,bool(0));
  rep(i,0,m){
    ll a,b;cin>>a>>b;a--;b--;
    g[a][b]=true;
    g[b][a]=true;
  }
  auto gr=g;
  rep(i,0,n)rep(j,0,n){
    if(i!=j)gr[i][j]=!gr[i][j];
  }
  //debug(g,n,n);
  //debug(gr,n,n);
  vector<P>p;
  rep(i,0,n){
    if(used[i]==-1){
      vector<ll>cnt(2);
      dfs(i,cnt,gr,0);
      p.EB(cnt[0],cnt[1]);
      //cout<<cnt[0] spa cnt[1] spa judgebuf<<endl;
    }
  }
  if(!judgebuf)cout<<-1<<endl;
  else{
    ll sz=p.size();
    auto dp=vec(sz+1,n+1,false);
    dp[0][0]=true;
    rep(i,0,sz){
      rep(j,0,n+1){
        if(dp[i][j]){
          dp[i+1][j+p[i].fi]=true;
          dp[i+1][j+p[i].se]=true;
        }
      }
    }
    ll ret=INF;
    rep(i,0,n+1){
      if(!dp[sz][i])continue;
      chmin(ret,i*(i-1)/2+(n-i)*(n-i-1)/2);
    }
    cout<<ret<<endl;
  }
  return 0;
}

Measured memory_bytes: 512