#define _USE_MATH_DEFINES
#include <iostream>
#include <fstream>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cstring>
#include <cassert>
#include <string>
#include <vector>
#include <utility>
#include <complex>
#include <set>
#include <map>
#include <queue>
#include <stack>
#include <deque>
#include <tuple>
#include <bitset>
#include <limits>
#include <algorithm>
#include <array>
#include <random>
#include <complex>
#include <regex>
using namespace std;
typedef long double ld;
typedef long long ll;
typedef vector<int> vint;
typedef vector<ll> vll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef pair<double, double> pdd;
typedef complex<ld> compd;
#define quickIO()	{cin.tie(0);	cout.sync_with_stdio(false);}
#define reach(i,a)	for(auto i:a)
#define rep(i,n)	for(int i=0;i<((int)n);i++)
#define REP(i,n)	for(int i=0;i<=((int)n);i++)
#define srep(i,a,n)	for(int i=a;i<((int)n);i++)
#define SREP(i,a,n)	for(int i=a;i<=((int)n);i++)
#define rrep(i,n)	for(int i=n-1;i>=0;i--)
#define RREP(i,n)	for(int i=n;i>=0;i--)
#define all(a)	(a).begin(),(a).end()
#define mp(a,b)	make_pair(a,b)
#define mt	make_tuple
#define pb	push_back
template<typename T>
istream& operator >> (istream& is, vector<T>& vec) {
	for (T& x : vec)	is >> x;
	return is;
}
template<typename T>
ostream& operator << (ostream& os, vector<T>& vec) {
	rep(i, vec.size())	os << (i ? " " : "") << vec[i];
	return os;
}
int bitcnt(ll x) {
	x = ((x & 0xAAAAAAAAAAAAAAAA) >> 1) + (x & 0x5555555555555555);
	x = ((x & 0xCCCCCCCCCCCCCCCC) >> 2) + (x & 0x3333333333333333);
	x = ((x & 0xF0F0F0F0F0F0F0F0) >> 4) + (x & 0x0F0F0F0F0F0F0F0F);
	x = ((x & 0xFF00FF00FF00FF00) >> 8) + (x & 0x00FF00FF00FF00FF);
	x = ((x & 0xFFFF0000FFFF0000) >> 16) + (x & 0x0000FFFF0000FFFF);
	x = ((x & 0xFFFFFFFF00000000) >> 32) + (x & 0x00000000FFFFFFFF);
	return x;
}
int bitcnt(int x) {
	x = ((x & 0xAAAAAAAA) >> 1) + (x & 0x55555555);
	x = ((x & 0xCCCCCCCC) >> 2) + (x & 0x33333333);
	x = ((x & 0xF0F0F0F0) >> 4) + (x & 0x0F0F0F0F);
	x = ((x & 0xFF00FF00) >> 8) + (x & 0x00FF00FF);
	x = ((x & 0xFFFF0000) >> 16) + (x & 0x0000FFFF);
	return x;
}
ll sqrtll(ll x) {
	ll left = 0, right = x;
	rep(i, 100) {
		ll mid = (left + right) >> 1;
		if (mid*mid <= x)	left = mid;
		else	right = mid;
	}
	return left;
}
ll gcd(ll a, ll b) {
	return a % b == 0 ? b : gcd(b, a%b);
}
#define debug(x)	printf("Case #%d: ", x)
#define DEBUG 0
const ll inf = 1LL << 60;
const ld infl = 1e100;
const ll mod = 1e9 + 7;
const ld eps = 1e-9;
const int dx[] = { 1,0,-1,0,0 };
const int dy[] = { 0,1,0,-1,0 };

bool edge[1000][1000];
int color[1000];
int n;
bool dp[1000], dp2[1000];

bool dfs(int cur, int c) {
	color[cur] = c;
	rep(i, n) {
		if (edge[cur][i]) {
			if (color[i] == c)	return false;
			if (color[i] == -1) {
				if (!dfs(i, c ^ 1))	return false;
			}
		}
	}
	return true;
}

int main() {
	int m;	cin >> n >> m;
	rep(i, n)	rep(j, n)	edge[i][j] = i != j;
	rep(i, m) {
		int a, b;	cin >> a >> b;	a--;	b--;
		edge[a][b] = edge[b][a] = false;
	}
	// 2部グラフ
	rep(i, n)	color[i] = -1;
	int cnt = 0;
	rep(i, n) {
		if (color[i] == -1) {
			if (!dfs(i, cnt)) {
				cout << -1 << endl;
				return 0;
			}
			cnt += 2;
		}
	}
	dp[0] = true;
	map<int, int> mp;
	rep(i, n)	mp[color[i]]++;
	rep(i, cnt >> 1) {
		int cnt1 = mp[i << 1];
		int cnt2 = mp[(i << 1) + 1];
		RREP(i, n) {
			if (i + cnt1 <= n)	dp2[i + cnt1] |= dp[i];
			if (i + cnt2 <= n)	dp2[i + cnt2] |= dp[i];
		}
		RREP(i, n) {
			dp[i] = dp2[i];
			dp2[i] = false;
		}
	}
	int ret = 1 << 25;
	REP(i, n) {
		if (dp[i] && dp[n-i]) {
			int j = n - i;
			ret = min(ret, i*(i - 1) / 2 + j * (j - 1) / 2);
		}
	}
	cout << ret << endl;
	return 0;
}

Measured memory_bytes: 1024