/*
    _____
  .'     '.
 /  0   0  \
|     ^     |
|  \     /  |
 \  '---'  /
  '._____.'
*/
#include <bits/stdc++.h>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/assoc_container.hpp>

using namespace std;
using namespace __gnu_pbds;

struct chash
{
	int operator()(int x) const
	{
		x ^= (x >> 20) ^ (x >> 12);
		return x ^ (x >> 7) ^ (x >> 4);
	}
};

template<typename T> using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
template<typename T, typename U> using hashtable = gp_hash_table<T, U, chash>;
random_device(rd);
mt19937 rng(rd());

template<class T>
void readi(T &x)
{
	T input = 0;
	bool negative = false;
	char c = ' ';
	while (c < '-')
	{
		c = getchar();
	}
	if (c == '-')
	{
		negative = true;
		c = getchar();
	}
	while (c >= '0')
	{
		input = input * 10 + (c - '0');
		c = getchar();
	}
	if (negative)
	{
		input = -input;
	}
	x = input;
}
template<class T>
void printi(T output)
{
	if (output == 0)
	{
		putchar('0');
		return;
	}
	if (output < 0)
	{
		putchar('-');
		output = -output;
	}
	int aout[20];
	int ilen = 0;
	while(output)
	{
		aout[ilen] = ((output % 10));
		output /= 10;
		ilen++;
	}
	for (int i = ilen - 1; i >= 0; i--)
	{
		putchar(aout[i] + '0');
	}
	return;
}
template<class T>
void ckmin(T &a, T b)
{
	a = min(a, b);
}
template<class T>
void ckmax(T &a, T b)
{
	a = max(a, b);
}
template<class T, class U>
T nmod(T &x, U mod)
{
	if (x >= mod) x -= mod;
}
template<class T>
T randomize(T mod)
{
	return (uniform_int_distribution<T>(0, mod - 1))(rng);
}

#define y0 ___y0
#define y1 ___y1
#define MP make_pair
#define MT make_tuple
#define PB push_back
#define PF push_front
#define LB lower_bound
#define UB upper_bound
#define fi first
#define se second
#define debug(x) cerr << #x << " = " << x << endl;

const long double PI = 4.0 * atan(1.0);
const long double EPS = 1e-10;

#define MAGIC 347
#define SINF 10007
#define CO 1000007
#define INF 1000000007
#define BIG 1000000931
#define LARGE 1696969696967ll
#define GIANT 2564008813937411ll
#define LLINF 2696969696969696969ll
#define MAXN 713

typedef long long ll;
typedef long double ld;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef pair<ld, ld> pdd;

int N, M;
bitset<MAXN> edge[MAXN];
int dsu[MAXN], sz[MAXN];
vector<pii> sizes;
bitset<MAXN> color, vis, dp;
int freq[2];
int ans = INF;

void init_dsu()
{
	for (int i = 0; i < N; i++)
	{
		dsu[i] = i;
		sz[i] = 1;
	}
}
int find_parent(int u)
{
	return (u == dsu[u] ? u : dsu[u] = find_parent(dsu[u]));
}
bool merge(int u, int v)
{
	u = find_parent(u);
	v = find_parent(v);
	if (u == v)
	{
		return false;
	}
	if (sz[u] > sz[v])
	{
		swap(u, v);
	}
	dsu[u] = v;
	sz[v] += sz[u];
	sz[u] = 0;
	return true;
}
bool dfs(int u, bool col)
{
	if (vis[u])
	{
		if (col != color[u])
		{
			return false;
		}
		return true;
	}
	vis[u] = true;
	color[u] = col;
	freq[color[u]]++;
	for (int v = 0; v < N; v++)
	{
		if (!edge[u][v]) continue;
		if (!dfs(v, !col)) return false;
	}
}
int choose2(int x)
{
	return x * (x - 1) / 2;
}

int32_t main()
{
	ios_base::sync_with_stdio(0);
	// cout << fixed << setprecision(10);
	// cerr << fixed << setprecision(10);
	// freopen ("file.in", "r", stdin);
	// freopen ("file.out", "w", stdout);
	cin >> N >> M;
	for (int i = 0; i < M; i++)
	{
		int u, v;
		cin >> u >> v;
		u--; v--;
		edge[u][v] = true;
		edge[v][u] = true;
	}
	for (int i = 0; i < N; i++)
	{
		for (int j = 0; j < N; j++)
		{
			edge[i][j] = edge[i][j] ^ true;
		}
		edge[i][i] = false;
	}
	init_dsu();
	for (int i = 0; i < N; i++)
	{
		for (int j = 0; j < N; j++)
		{
			if (edge[i][j])
			{
				merge(i, j);
			}
		}
	}
	for (int i = 0; i < N; i++)
	{
		if (find_parent(i) != i)
		{
			continue;
		}
		if (!dfs(i, 0))
		{
			cout << "-1\n";
			return 0;
		}
		if (freq[0] < freq[1])
		{
			swap(freq[0], freq[1]);
		}
		sizes.PB({freq[0], freq[1]});
		freq[0] = 0; freq[1] = 0;
	}
	dp[0] = true;
	for (pii p : sizes)
	{
		dp = dp << (p.se);
		dp |= (dp << (p.fi - p.se));
	}
	for (int i = 0; i <= N; i++)
	{
		if (dp[i])
		{
			ckmin(ans, choose2(i) + choose2(N - i));
		}
	}
	cout << ans << '\n';
	// cerr << "time elapsed = " << (clock() / (CLOCKS_PER_SEC / 1000)) << " ms" << endl;
	return 0;
}


Measured memory_bytes: 384