#include <bits/stdc++.h>

using namespace std;

using ll = long long;
using ull = unsigned long long;

#define REP(i,n) for(long long i = 0; i < (n); i++)
#define FOR(i, m, n) for(long long i = (m);i < (n); ++i)
#define ALL(obj) (obj).begin(),(obj).end()

template<class T> using V = vector<T>;
template<class T, class U> using P = pair<T, U>;

const ll MOD = (ll)1e9 + 7;
const ll MOD2 = 998244353;
const ll LLINF = (ll)1e18;
const ll INTINF = (ll)1e9;
const long double PI = 3.1415926535897932384626433;

template <class T> void corner(bool flg, T hoge) {if (flg) {cout << hoge << endl; exit(0);}}
template <class T, class U>ostream &operator<<(ostream &o, const map<T, U>&obj) {o << "{"; for (auto &x : obj) o << " {" << x.first << " : " << x.second << "}" << ","; o << " }"; return o;}
template <class T>ostream &operator<<(ostream &o, const set<T>&obj) {o << "{"; for (auto itr = obj.begin(); itr != obj.end(); ++itr) o << (itr != obj.begin() ? ", " : "") << *itr; o << "}"; return o;}
template <class T>ostream &operator<<(ostream &o, const multiset<T>&obj) {o << "{"; for (auto itr = obj.begin(); itr != obj.end(); ++itr) o << (itr != obj.begin() ? ", " : "") << *itr; o << "}"; return o;}
template <class T>ostream &operator<<(ostream &o, const vector<T>&obj) {o << "{"; for (int i = 0; i < (int)obj.size(); ++i)o << (i > 0 ? ", " : "") << obj[i]; o << "}"; return o;}
template <class T, class U>ostream &operator<<(ostream &o, const pair<T, U>&obj) {o << "{" << obj.first << ", " << obj.second << "}"; return o;}
template <template <class tmp>  class T, class U> ostream &operator<<(ostream &o, const T<U> &obj) {o << "{"; for (auto itr = obj.begin(); itr != obj.end(); ++itr)o << (itr != obj.begin() ? ", " : "") << *itr; o << "}"; return o;}
void print(void) {cout << endl;}
template <class Head> void print(Head&& head) {cout << head;print();}
template <class Head, class... Tail> void print(Head&& head, Tail&&... tail) {cout << head << " ";print(forward<Tail>(tail)...);}
template <class T> void chmax(T& a, const T b){a=max<T>(a,b);}
template <class T> void chmin(T& a, const T b){a=min<T>(a,b);}
void YN(bool flg) {cout << ((flg) ? "YES" : "NO") << endl;}
void Yn(bool flg) {cout << ((flg) ? "Yes" : "No") << endl;}
void yn(bool flg) {cout << ((flg) ? "yes" : "no") << endl;}

int main() {
	int N,M; cin >> N >> M;
	corner(M==0 && N == 2,0);
	corner(M==0 && N > 2,-1);
	
	V<V<int>> flg(N,V<int>(N,1));
	for(int i = 0; i < N; ++i) flg[i][i] = 0;
	while(M--){
		int A,B; cin >> A >> B;
		A--,B--;
		flg[A][B] = flg[B][A] = 0;
	}
	V<V<int>> edge(N);
	for(int i = 0; i < N; ++i) {
		for(int j = 0; j < N; ++j){
			if(flg[i][j]) edge[i].push_back(j);
		}
	}
	V<int> g(N,-1);
	int k = 0;
	for(int i = 0; i < N; ++i){
		if(edge[i].empty()) continue;
		if(g[i]>=0) continue;
		
		queue<P<int,int>> q;
		g[i] = k;
		q.push({i,0});
		while(q.size()){
			int from = q.front().first;
			int prt = q.front().second;
			q.pop();
			for(auto to: edge[from]){
				if(g[to] == -1){
					g[to] = k + !prt;
					q.push({to,!prt});
				}
				else corner(g[to] != (k + !prt),-1); 
			}
		}
		k += 2;
	}
	int p = 0;
	k /= 2;
	V<V<int>> c(k,V<int>(2,0));
	for(int i = 0; i < N; ++i){
		if(g[i] == -1) p++;
		else c[g[i]/2][g[i]%2]++;
	}
	int sum = 0;
	for(int i = 0; i < k; ++i) sum += min(c[i][0],c[i][1]);
	V<int> dp(N+1,0);
	dp[sum]++;
	V<int> d(k);
	for(int i = 0; i < k; ++i) d[i] = abs(c[i][0]-c[i][1]);
	for(int i = 0; i < k; ++i) for(int j = N; 0 <= j; --j) if(dp[j] && j + d[i] <= N) dp[j + d[i]] = 1;
	for(int i = 0; i < p; ++i) for(int j = N; 0 <= j; --j) if(dp[j] && j + 1 <= N) dp[j + 1] = 1;
	int a = N + 1,b;
	for(int i = 0; i <= N; ++i) if(dp[i] && abs(a-N/2) > abs(i - N/2)) a = i;
	b = N - a;
	cout << (a*(a-1)+b*(b-1))/2 << endl;	
	return 0;
}


Measured memory_bytes: 2816