#include <bits/stdc++.h>

#define FOR(i,a,b) for(int i= (a); i<((int)b); ++i)
#define RFOR(i,a) for(int i=(a); i >= 0; --i)
#define FOE(i,a) for(auto i : a)
#define ALL(c) (c).begin(), (c).end()
#define RALL(c) (c).rbegin(), (c).rend()
#define DUMP(x)  cerr << #x << " = " << (x) << endl;
#define SUM(x) std::accumulate(ALL(x), 0LL)
#define MIN(v) *std::min_element(v.begin(), v.end())
#define MAX(v) *std::max_element(v.begin(), v.end())
#define EXIST(v,x) (std::find(v.begin(), v.end(), x) != v.end())
#define BIT(n) (1LL<<(n))
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end());

typedef long long LL;
template<typename T> using V = std::vector<T>;
template<typename T> using VV = std::vector<std::vector<T>>;
template<typename T> using VVV = std::vector<std::vector<std::vector<T>>>;

template<class T> inline T ceil(T a, T b) { return (a + b - 1) / b; }
template<class T> inline void print(T x) { std::cout << x << std::endl; }
template<class T> inline void print_vec(const std::vector<T> &v) { for (int i = 0; i < v.size(); ++i) {  if (i != 0) {std::cout << " ";} std::cout << v[i];} std::cout << "\n"; }
template<class T> inline bool inside(T y, T x, T H, T W) {return 0 <= y and y < H and 0 <= x and x < W; }
template<class T> inline double euclidean_distance(T y1, T x1, T y2, T x2) { return sqrt((x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2)); }
template<class T> inline double manhattan_distance(T y1, T x1, T y2, T x2) { return abs(x1 - x2) + abs(y1 - y2); }

const int INF = 1L << 30;
const double EPS = 1e-9;
const std::string YES = "YES", Yes = "Yes", NO = "NO", No = "No";
const std::vector<int> dy4 = { 0, 1, 0, -1 }, dx4 = { 1, 0, -1, 0 };    // 4近傍（右, 下, 左, 上）
const std::vector<int> dy8 = { 0, -1, 0, 1, 1, -1, -1, 1 }, dx8 = { 1, 0, -1, 0, 1, 1, -1, -1 };

using namespace std;

// 頂点を1と-1で塗っていく
bool dfs(int node, int color, vector<int> &colors, vector<bool> &used, const vector<vector<int>> &graph) {
    colors[node] = color;
    used[node] = true;

    for (int i = 0; i < graph[node].size(); ++i) {
        int next_node = graph[node][i];

        // 隣のノードが同じ色
        if (colors[next_node] == color) { return false; }

            // 隣のノードは未着色
        else if (colors[next_node] == 0) {
            if (dfs(next_node, -color, colors, used, graph) == false) {
                return false;
            }
        }
    }
    return true;
}

int main() {
    cin.tie(0);
	ios::sync_with_stdio(false);

	int N, M;
	cin >> N >> M;
	vector<vector<bool>> graph(N, vector<bool>(N, false));
	FOR(i, 0, M) {
	    int A, B;
	    cin >> A >> B;
	    A--; B--;
	    graph[A][B] = true;
	    graph[B][A] = true;
	}

    vector<vector<int>> comp_graph(N);
    FOR(i, 0, N) {
	    FOR(j, i + 1, N) {
	        if (not graph[i][j]) {
                comp_graph[i].emplace_back(j);
                comp_graph[j].emplace_back(i);
	        }
	    }
	}

    // 各ノードの色の管理(0は未着色)
    vector<pair<int, int>> v;
    vector<bool> used(N, false);
    for (int i = 0; i < graph.size(); i++) {
        if (not used[i]) {
            vector<int> colors(graph.size(), 0);
            if (!dfs(i, 1, colors, used, comp_graph)) {
                print(-1);
                return 0;
            }
            int a = 0, b = 0;
            FOR(i, 0, colors.size()) {
                a += (colors[i] == 1);
                b += (colors[i] == -1);
            }
            v.emplace_back(make_pair(a, b));
        }
    }

    vector<vector<bool>> dp(v.size() + 1, vector<bool>(1000, false));
    dp[0][0] = true;
    FOR(i, 0, v.size()) {
        FOR(j, 0, dp[0].size()) {
            if (dp[i][j]) {
                dp[i + 1][j + v[i].first] = true;
                dp[i + 1][j + v[i].second] = true;
            }
        }
    }

    int ans = INT_MAX;
    FOR(j, 0, dp[0].size()) {
        if (dp[v.size()][j]) {
            int s = j;
            int t = N - s;
            ans = min(ans, (s * (s - 1) / 2) + (t * (t - 1) / 2));
        }
    }
    print(ans);

	return 0;
}

Measured memory_bytes: 3200