// need
#include <iostream>
#include <algorithm>

// data structure
#include <bitset>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <utility>
#include <vector>
#include <complex>
//#include <deque>
#include <valarray>
#include <array>
#include <unordered_map>

// stream
//#include <istream>
//#include <sstream>
//#include <ostream>
#include <fstream>

// etc
#include <cassert>
#include <cmath>
#include <functional>
#include <iomanip>
#include <chrono>
#include <random>
#include <numeric>

// input
#define INIT std::ios::sync_with_stdio(false);std::cin.tie(0);
#define VAR(type, ...)type __VA_ARGS__;MACRO_VAR_Scan(__VA_ARGS__);
template<typename T> void MACRO_VAR_Scan(T& t) { std::cin >> t; }
template<typename First, typename...Rest>void MACRO_VAR_Scan(First& first, Rest&...rest) { std::cin >> first; MACRO_VAR_Scan(rest...); }
#define VEC_ROW(type, n, ...)std::vector<type> __VA_ARGS__;MACRO_VEC_ROW_Init(n, __VA_ARGS__); for(int i=0; i<n; ++i){MACRO_VEC_ROW_Scan(i, __VA_ARGS__);}
template<typename T> void MACRO_VEC_ROW_Init(int n, T& t) { t.resize(n); }
template<typename First, typename...Rest>void MACRO_VEC_ROW_Init(int n, First& first, Rest&...rest) { first.resize(n); MACRO_VEC_ROW_Init(n, rest...); }
template<typename T> void MACRO_VEC_ROW_Scan(int p, T& t) { std::cin >> t[p]; }
template<typename First, typename...Rest>void MACRO_VEC_ROW_Scan(int p, First& first, Rest&...rest) { std::cin >> first[p]; MACRO_VEC_ROW_Scan(p, rest...); }
#define VEC(type, c, n) std::vector<type> c(n);for(auto& i:c)std::cin>>i;
#define MAT(type, c, m, n) std::vector<std::vector<type>> c(m, std::vector<type>(n));for(auto& r:c)for(auto& i:r)std::cin>>i;

// output
#define OUT(d) std::cout<<(d);
#define FOUT(n, d) std::cout<<std::fixed<<std::setprecision(n)<<(d);
#define SOUT(n, c, d) std::cout<<std::setw(n)<<std::setfill(c)<<(d);
#define SP std::cout<<" ";
#define TAB std::cout<<"\t";
#define BR std::cout<<"\n";
#define SPBR(i, n) std::cout<<(i + 1 == n ? '\n' : ' ');
#define ENDL std::cout<<std::endl;
#define FLUSH std::cout<<std::flush;
#define SHOW(d) {std::cerr << #d << "\t:" << (d) << "\n";}
#define SHOWVECTOR(v) {std::cerr << #v << "\t:";for(const auto& xxx : v){std::cerr << xxx << " ";}std::cerr << "\n";}
#define SHOWVECTOR2(v) {std::cerr << #v << "\t:\n";for(const auto& xxx : v){for(const auto& yyy : xxx){std::cerr << yyy << " ";}std::cerr << "\n";}}
#define SHOWQUEUE(a) {auto tmp(a);std::cerr << #a << "\t:";while(!tmp.empty()){std::cerr << tmp.front() << " ";tmp.pop();}std::cerr << "\n";}

// utility
#define ALL(a) (a).begin(),(a).end()
#define FOR(i, a, b) for(int i=(a);i<(b);++i)
#define RFOR(i, a, b) for(int i=(b)-1;i>=(a);--i)
#define REP(i, n) for(int i=0;i<int(n);++i)
#define RREP(i, n) for(int i=int(n)-1;i>=0;--i)
#define FORLL(i, a, b) for(ll i=ll(a);i<ll(b);++i)
#define RFORLL(i, a, b) for(ll i=ll(b)-1;i>=ll(a);--i)
#define REPLL(i, n) for(ll i=0;i<ll(n);++i)
#define RREPLL(i, n) for(ll i=ll(n)-1;i>=0;--i)
#define IN(a, x, b) (a<=x && x<b)
template<typename T> inline T CHMAX(T& a, const T b) { return a = (a < b) ? b : a; }
template<typename T> inline T CHMIN(T& a, const T b) { return a = (a > b) ? b : a; }
#define EXCEPTION(msg) throw std::string("Exception : " msg " [ in ") + __func__ + " : " + std::to_string(__LINE__) + " lines ]"
#define TRY(cond, msg) try {if (cond) EXCEPTION(msg);}catch (std::string s) {std::cerr << s << std::endl;}
void CHECKTIME(std::function<void()> f) { auto start = std::chrono::system_clock::now(); f(); auto end = std::chrono::system_clock::now(); auto res = std::chrono::duration_cast<std::chrono::nanoseconds>((end - start)).count(); std::cerr << "[Time:" << res << "ns  (" << res / (1.0e9) << "s)]\n"; }

// test
template<class T> std::vector<std::vector<T>> VV(int n, int m, T init = T()) {
	return std::vector<std::vector<T>>(n, std::vector<T>(m, init));
}
template<typename S, typename T>
std::ostream& operator<<(std::ostream& os, std::pair<S, T> p) {
	os << "(" << p.first << ", " << p.second << ")"; return os;
}

// type/const
#define int ll
using ll = long long;
using ull = unsigned long long;
using ld = long double;
using PAIR = std::pair<int, int>;
using PAIRLL = std::pair<ll, ll>;
constexpr int INFINT = 1 << 30;                          // 1.07x10^ 9
constexpr int INFINT_LIM = (1LL << 31) - 1;              // 2.15x10^ 9
constexpr ll INFLL = 1LL << 60;                          // 1.15x10^18
constexpr ll INFLL_LIM = (1LL << 62) - 1 + (1LL << 62);  // 9.22x10^18
constexpr double EPS = 1e-9;
constexpr int MOD = 1000000007;
constexpr double PI = 3.141592653589793238462643383279;

template<class T, size_t N> void FILL(T(&a)[N], const T& val) { for (auto& x : a) x = val; }
template<class ARY, size_t N, size_t M, class T> void FILL(ARY(&a)[N][M], const T& val) { for (auto& b : a) FILL(b, val); }
template<class T> void FILL(std::vector<T>& a, const T& val) { for (auto& x : a) x = val; }
template<class ARY, class T> void FILL(std::vector<std::vector<ARY>>& a, const T& val) { for (auto& b : a) FILL(b, val); }

// ------------>8------------------------------------->8------------

int num(int sz) {
	return sz * (sz - 1) / 2;
}

signed main() {
	INIT;

	VAR(int, n, m);
	VEC_ROW(int, m, a, b);

	std::vector<std::vector<int>> g(n, std::vector<int>(n, 0));
	std::vector<std::vector<int>> h(n, std::vector<int>(n, 0));

	REP(i, m) {
		--a[i]; --b[i];
		g[a[i]][b[i]] = g[b[i]][a[i]] = 1;
	}

	REP(i, n) FOR(j, i + 1, n) FOR(k, j + 1, n) {
		int t = g[i][j] + g[j][k] + g[k][i];
		if (t == 0) {
			OUT(-1)BR;
			return 0;
		}
	}

	REP(i, n) REP(j, n) if (i != j) {
		h[i][j] = 1 - g[i][j];
	}

	/*SHOWVECTOR2(g);
	SHOWVECTOR2(h);

	BR
	REP(i, n) FOR(j, i + 1, n) {
		if (h[i][j]) {
			OUT(i)SP OUT(j)BR;
		}
	}*/

	std::vector<int> col(n, -1);

	std::vector<PAIR> v;

	REP(i, n) {
		if (col[i] != -1) continue;
		std::queue<int> que({ i });
		col[i] = 0;

		int cnt[2] = {1, 0};
		while (!que.empty()) {
			int now = que.front(); que.pop();
			REP(to, n) if(h[now][to]) {
				if (col[to] != -1) continue;
				++cnt[col[to] = 1 - col[now]];
				que.push(to);
			}
		}
		v.emplace_back(PAIR(cnt[0], cnt[1]));
	}

	auto check = [&](int d) -> bool {
		std::vector<std::vector<std::vector<bool>>> dp(v.size() + 1, std::vector<std::vector<bool>>(d + 1, std::vector<bool>(d + 1, false)));
		dp[0][0][0] = true;
		REP(i, v.size()) REP(j, d + 1) REP(k, d + 1){
			if (!dp[i][j][k]) continue;
			if (j + v[i].first <= d && k + v[i].second <= d) dp[i + 1][j + v[i].first][k + v[i].second] = true;
			if (j + v[i].second <= d && k + v[i].first <= d) dp[i + 1][j + v[i].second][k + v[i].first] = true;
		}
		bool res = false;
		REP(j, d + 1) REP(k, d + 1) res |= dp[v.size()][j][k];
		return res;
	};
	int ng = 0, ok = n;
	while (ok - ng > 1) {
		int md = (ok + ng) / 2;
		if (check(md)) ok = md;
		else ng = md;
	}
	OUT(num(ok) + num(n - ok))BR;
	return 0;
}

/*

答えは i(i-1)/2 + (n-i)(n-i-1)/2 の形
(もしくは -1)

ある3点がどれも互いに隣接していなければ-1

*/

Measured memory_bytes: 54912