#include <bits/stdc++.h>
using namespace std;
#define rep(i, n)  for(long long i=0;i<(long long)(n);i++)
#define REP(i,k,n) for(long long i=k;i<(long long)(n);i++)
#define all(a) a.begin(),a.end()
#define pb push_back
#define eb emplace_back
#define lb(v,k) (lower_bound(all(v),k)-v.begin())
#define ub(v,k) (upper_bound(all(v),k)-v.begin())
#define fi first
#define se second
#define pi M_PI
#define PQ(T) priority_queue<T>
#define SPQ(T) priority_queue<T,vector<T>,greater<T>>
#define dame(a) {out(a);return 0;}
#define decimal cout<<fixed<<setprecision(15);
typedef long long ll;
typedef pair<ll,ll> P;
typedef tuple<ll,ll,ll> PP;
typedef tuple<ll,ll,ll,ll> PPP;
typedef multiset<ll> S;
using vi=vector<ll>;
using vvi=vector<vi>;
using vvvi=vector<vvi>;
using vp=vector<P>;
using vvp=vector<vp>;
using vb=vector<bool>;
using vvb=vector<vb>;
const ll inf=1001001001001001001;
const int INF=1001001001;
const int mod=998244353;
const double eps=1e-10;
template<class T> bool chmin(T&a,T b){if(a>b){a=b;return true;}return false;}
template<class T> bool chmax(T&a,T b){if(a<b){a=b;return true;}return false;}
template<class T> void out(T a){cout<<a<<'\n';}
template<class T> void outp(T a){cout<<'('<<a.fi<<','<<a.se<<')'<<'\n';}
template<class T> void outvp(T v){rep(i,v.size())cout<<'('<<v[i].fi<<','<<v[i].se<<')';cout<<'\n';}
template<class T> void outvvp(T v){rep(i,v.size())outvp(v[i]);}
template<class T> void outv(T v){rep(i,v.size()){if(i)cout<<' ';cout<<v[i];}cout<<'\n';}
template<class T> void outvv(T v){rep(i,v.size())outv(v[i]);}
template<class T> bool isin(T x,T l,T r){return (l)<=(x)&&(x)<=(r);}
template<class T> void yesno(T b){if(b)out("yes");else out("no");}
template<class T> void YesNo(T b){if(b)out("Yes");else out("No");}
template<class T> void YESNO(T b){if(b)out("YES");else out("NO");}
template<class T> void noyes(T b){if(b)out("no");else out("yes");}
template<class T> void NoYes(T b){if(b)out("No");else out("Yes");}
template<class T> void NOYES(T b){if(b)out("NO");else out("YES");}
void outs(ll a,ll b){if(a>=inf-100)out(b);else out(a);}
ll gcd(ll a,ll b){if(b==0)return a;return gcd(b,a%b);}
ll modpow(ll a,ll b){a%=mod;if(b==0)return 1;if(b&1)return a*modpow(a,b-1)%mod;ll k=modpow(a,b/2);return k*k%mod;}
vi color;
vvi g;
P res;
bool is_nibu=true;
void dfs(int i,int c){
    if(!is_nibu)return;
    if(color[i]==c)return;
    if(color[i]!=-1){
        is_nibu=false;
        return;
    }
    if(c)res.fi++;
    else res.se++;
    color[i]=c;
    for(auto x:g[i])if(color[x]!=1-c)dfs(x,1-c);
}
int main(){
    ll n,m;cin>>n>>m;
    vvb v(n,vb(n,true));
    g=vvi(n);
    rep(i,m){
        ll a,b;cin>>a>>b;a--;b--;
        if(a<b)swap(a,b);
        v[a][b]=false;
    }
    rep(i,n)rep(j,i)if(v[i][j]){
        g[i].pb(j);
        g[j].pb(i);
    }
    color=vi(n,-1);
    vp d;
    rep(i,n)if(color[i]==-1){
        res=P(0,0);
        dfs(i,0);
        if(res.fi>res.se)swap(res.fi,res.se);
        d.eb(res);
    }
    if(!is_nibu)dame(-1);
    vb dp(n/2+1,false);
    dp[0]=true;
    for(auto x:d){
        vb ndp(n/2+1,false);
        for(int i=dp.size()-1;i>0;i--)if(!ndp[i]){
            if(i-x.fi>=0)if(dp[i-x.fi])ndp[i]=true;
            if(i-x.se>=0)if(dp[i-x.se])ndp[i]=true;
        }
        dp=ndp;
    }
    ll ans=dp.size()-1;
    while(!dp[ans])ans--;
    out(n*(n-1)/2-ans*(n-ans));
}

Measured memory_bytes: 6016