// >>> TEMPLATES
#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for (int i = 0; i < int(n); i++)
#define rep1(i,n) for (int i = 1; i <= int(n); i++)
#define repR(i,n) for (int i = int(n)-1; i >= 0; i--)
#define rep1R(i,n) for (int i = int(n); i >= 1; i--)
#define loop(i,a,B) for (int i = a; i B; i++)
#define loopR(i,a,B) for (int i = a; i B; i--)
#define all(x) (x).begin(), (x).end()
#define allR(x) (x).rbegin(), (x).rend()
#define eb emplace_back
#define mp make_pair
#define fst first
#define snd second
#ifdef LOCAL
#define dump(...) cerr << "[" << __LINE__ << ":" << __FUNCTION__ << "] ", my_dmp(#__VA_ARGS__, __VA_ARGS__)
#define say(x) cerr << "[" << __LINE__ << ":" << __FUNCTION__ << "] " << x << endl;
#define debug if (1)
void my_dmp(const char*) { cerr << endl; }
template <class T, class... U> void my_dmp(const char *s, T const& x, U const& ...y) { const char *o = "({[", *e = "]})"; for (int i = 0; *s != '\0'; cerr << *s++) { if (count(o,o+3,*s)) i++; if (count(e,e+3,*s)) i--; if (!i && *s == ',') break; } cerr << " = " << x; if (*s == ',') cerr << ", ", s++; my_dmp(s, y...); }
#else
#define dump(...)
#define say(x)
#define debug if (0)
#endif
using ll = long long;
using ld = long double;
#define int ll
#define double ld
template <class T> using pque_max = priority_queue<T>;
template <class T> using pque_min = priority_queue<T, vector<T>, greater<T> >;
template <class T, class = typename T::iterator, class = typename enable_if<!is_same<T, string>::value>::type>
ostream& operator<<(ostream& os, T const& v) { os << "{"; for (auto const& x : v) os << " " << x; return os << " }"; }
template <class T> istream& operator>>(istream& is, vector<T>& v) { for (auto& x : v) is >> x; return is; }
template <class T, class S> ostream& operator<<(ostream& os, pair<T,S> const& p) { return os << "(" << p.first << ", " << p.second << ")"; }
template <class T, class S> istream& operator>>(istream& is, pair<T,S>& p) { return is >> p.first >> p.second; }
template <size_t i, class T> typename enable_if<i >= tuple_size<T>::value>::type output_tuple(ostream&, T const&) { }
template <size_t i = 0, class T> typename enable_if<i < tuple_size<T>::value>::type
output_tuple(ostream& os, T const& t) { os << (i ? " " : "") << get<i>(t); output_tuple<i+1,T>(os,t); }
template <class... T> ostream& operator<<(ostream& os, tuple<T...> const& t) { return output_tuple(os,t), os; }
struct my_Init { my_Init() { cin.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(15); } } my_init;
template <class T, size_t d> struct vec_impl {
    using type = vector<typename vec_impl<T,d-1>::type>;
    template <class... U> static type make_v(size_t n, U&&... x) { return type(n, vec_impl<T,d-1>::make_v(forward<U>(x)...)); }
};
template <class T> struct vec_impl<T,0> { using type = T; static type make_v(T const& x = {}) { return x; } };
template <class T, size_t d> using vec = typename vec_impl<T,d>::type;
template <class T, size_t d, class... Args> auto make_v(Args&&... args) { return vec_impl<T,d>::make_v(forward<Args>(args)...); }
template <class T> void quit(T const& x) { cout << x << endl; exit(0); }
template <class T> constexpr bool chmin(T& x, T const& y) { if (x > y) { x = y; return true; } return false; }
template <class T> constexpr bool chmax(T& x, T const& y) { if (x < y) { x = y; return true; } return false; }
template <class It> constexpr auto sumof(It b, It e) { return accumulate(b,e,typename iterator_traits<It>::value_type{}); }
const ll INF = (1LL<<62)-1; // ~ 4.6e18
// <<<
// >>> union find
struct UnionFind {
    int n, sz; // id : 0...n-1
    vector<int> par;

    UnionFind(int n = 0) : n(n), sz(n), par(n,-1) { }
    int root(int x) {
        assert(0 <= x && x < n);
        return par[x] < 0 ? x : par[x] = root(par[x]);
    }
    bool unite(int x, int y) {
        x = root(x), y = root(y);
        if (x == y) return false;
        sz--;
        if (par[x] < par[y]) swap(x,y);
        par[y] += par[x];
        par[x] = y;
        return true;
    }
    bool same(int x, int y) { return root(x) == root(y); }
    int size(int x) { return -par[root(x)]; }
    int size() const { return sz; }
    struct groups_t {
        vector<vector<int> > grp;
        vector<pair<int,int> > id;
    };
    groups_t groups() {
        vector<vector<int> > g(n);
        rep (i,n) if (par[i] < 0) g[i].reserve(-par[i]);
        rep (i,n) g[root(i)].push_back(i);
        vector<vector<int> > grp; grp.reserve(size());
        rep (i,n) if (g[i].size()) grp.emplace_back(move(g[i]));
        vector<pair<int,int> > id(n);
        rep (i,grp.size()) rep (j,grp[i].size()) {
            id[grp[i][j]] = make_pair(i,j);
        }
        return { grp, id };
    }
};
// <<<

int32_t main() {
    int n,m; cin >> n >> m;
    auto g = make_v<int,2>(n,n,1);
    rep (i,m) {
        int a,b; cin >> a >> b; a--,b--;
        g[a][b] = g[b][a] = 0;
    }
    UnionFind uf(2*n);
    rep (i,n) rep (j,n) if (i != j && g[i][j]) {
        uf.unite(i,n+j);
        uf.unite(j,n+i);
    }
    rep (i,n) if (uf.same(i,n+i)) quit(-1);

    vector<int> used(n);
    vector<int> a;
    auto grp = uf.groups().grp;
    dump(grp);
    for (auto const& g : grp) {
        bool flag = false;
        int c1 = 0, c2 = 0;
        for (int x : g) {
            if (x >= n) x -= n, c2++; else c1++;
            if (used[x]) flag = true;
            used[x] = 1;
        }
        if (flag) continue;
        a.eb(abs(c1-c2));
    }

    int k = a.size();
    const int NN = n+5;
    auto dp = make_v<int,2>(k+1, 2*NN+1);
    dp[0][NN] = 1;
    rep (i,k) loop (j,-NN,<=NN) if (dp[i][NN+j]) {
        if (-NN <= j+a[i]  && j+a[i] <= NN) dp[i+1][NN+j+a[i]] = 1;
        if (-NN <= j-a[i]  && j-a[i] <= NN) dp[i+1][NN+j-a[i]] = 1;
    }
    //
    rep (i,k+1) dump(i,dp[i]);

    int mi = INF;
    loop (j,-NN,<=NN) if (dp[k][NN+j]) chmin(mi,abs(j));

    assert((n+mi)%2 == 0);
    int x = (n+mi)/2, y = (n-mi)/2;
    cout << x*(x-1)/2 + y*(y-1)/2 << endl;
    //
    dump(mi,x,y);
    dump(a);
}


Measured memory_bytes: 11904