#include <algorithm>
#include <bitset>
#include <climits>
#include <cmath>
#include <cstdio>
#include <iomanip>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <unordered_map>
#include <vector>

#define MOD 1000000007
#define MOD2 998244353
#define int long long
#define double long double
#define EPS 1e-9
//#define PI 3.14159265358979

#define rep(i, n) for (int i = 0; i < (int)(n); i++)

using namespace std;

template < typename T >
ostream &operator<<(ostream &os, const vector< T > &A) {
	for (int i = 0; i < A.size(); i++)
		os << A[i] << " ";
	os << endl;
	return os;
}
template <>
ostream &operator<<(ostream &os, const vector< vector< int > > &A) {
	int N = A.size();
	for (int i = 0; i < N; i++) {
		for (int j = 0; j < A[i].size(); j++)
			os << A[i][j] << " ";
		os << endl;
	}
	return os;
}

typedef pair< int, int > pii;
typedef long long ll;

struct edge {
	int from, to, d, c;
	edge(int _from = 0, int _to = 0, int _d = 0, int _c = 0) {
		from = _from;
		to = _to;
		d = _d;
		c = _c;
	}
	bool operator<(const edge &rhs) const {
		return (d == rhs.d) ? (c < rhs.c) : (d < rhs.d);
	}
};
struct aabb {
	int x1, y1, x2, y2;
	aabb(int x1, int y1, int x2, int y2) : x1(x1), y1(y1), x2(x2), y2(y2) {}
};

typedef vector< edge > edges;
typedef vector< edges > graph;
struct flow {
	int to, cap, rev, cost;
	flow(int to = 0, int cap = 0, int rev = 0, int cost = 0) : to(to), cap(cap), rev(rev), cost(cost) {}
};
typedef vector< vector< flow > > flows;

const int di[4] = {0, -1, 0, 1};
const int dj[4] = {-1, 0, 1, 0};
const int ci[5] = {0, 0, -1, 0, 1};
const int cj[5] = {0, -1, 0, 1, 0};
const ll LINF = LLONG_MAX / 2;
const int INF = INT_MAX / 2;
const double PI = acos(-1);

int pow2(int n) { return 1LL << n; }
template < typename T, typename U >
bool chmin(T &x, const U &y) {
	if (x > y) {
		x = y;
		return true;
	}
	return false;
}
template < typename T, typename U >
bool chmax(T &x, const U &y) {
	if (x < y) {
		x = y;
		return true;
	}
	return false;
}

struct initializer {
	initializer() {
		cout << fixed << setprecision(20);
	}
};
initializer _____;

int N, M, K, T, Q, H, W;
signed main() {
	cin >> N >> M;
	vector< vector< int > > G(N, vector< int >(N, 0));
	vector< vector< int > > H(N, vector< int >(N, 1));
	rep(i, M) {
		int a, b;
		cin >> a >> b;
		--a;
		--b;
		G[a][b] = 1;
		G[b][a] = 1;
		H[a][b] = 0;
		H[b][a] = 0;
	}
	rep(i, N) H[i][i] = 0;
	vector< int > L(N, -1);
	auto f = [&](auto &&f, int x, pii &p) -> int {
		rep(j, N) if (H[x][j]) {
			if (L[j] == L[x])
				return 1;
			if (L[j] != -1)
				continue;
			L[j] = 1 - L[x];
			if (L[j]) {
				p.second++;
			} else {
				p.first++;
			}
			if (f(f, j, p))
				return 1;
		}
		return 0;
	};
	vector< pii > P;
	rep(i, N) if (L[i] == -1) {
		L[i] = 0;
		pii p(1, 0);
		if (f(f, i, p)) {
			cout << -1 << endl;
			return 0;
		}
		P.push_back(p);
	}
	M = P.size();
	//rep(i, M) cout << P[i].first << " " << P[i].second << endl;
	vector< int > now(N + 1, 0), next(N + 1, 0);
	now[0] = 1;
	for (auto p : P) {
		int d = abs(p.second - p.first);
		rep(i, N + 1) if (now[i]) {
			next[abs(i - d)] = 1;
			next[i + d] = 1;
		}
		now = next;
		next.assign(N + 1, 0);
	}
	int m = 0, k = 0, din = LINF;
	rep(i, N + 1) if (now[i] && i < din) {
		din = i;
		m = (N - i) / 2;
		k = m + i;
	}
	cout << m * (m - 1) / 2 + k * (k - 1) / 2 << endl;
	return 0;
}

Measured memory_bytes: 7936