#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <iostream>
#include <memory.h>
#include <math.h>
#include <assert.h>
#include <queue>
#include <map>
#include <set>
#include <string>
#include <algorithm>
#include <functional>
#include <vector>
#include <stack>
#include <unordered_map>
using namespace std;

#define VA_NUM_ARGS(...) VA_NUM_ARGS_IMPL_((0,__VA_ARGS__, 6,5,4,3,2,1))
#define VA_NUM_ARGS_IMPL_(tuple) VA_NUM_ARGS_IMPL tuple
#define VA_NUM_ARGS_IMPL(_0,_1,_2,_3,_4,_5,_6,N,...) N
#define macro_dispatcher(macro, ...) macro_dispatcher_(macro, VA_NUM_ARGS(__VA_ARGS__))
#define macro_dispatcher_(macro, nargs) macro_dispatcher__(macro, nargs)
#define macro_dispatcher__(macro, nargs) macro_dispatcher___(macro, nargs)
#define macro_dispatcher___(macro, nargs) macro ## nargs
#define Debug1(a)           cout<<#a<<"="<<(a)<<"\n"
#define Debug2(a,b)         cout<<#a<<"="<<(a)<<", "<<#b<<"="<<(b)<<"\n"
#define Debug3(a,b,c)       cout<<#a<<"="<<(a)<<", "<<#b<<"="<<(b)<<", "<<#c<<"="<<(c)<<"\n"
#define Debug4(a,b,c,d)     cout<<#a<<"="<<(a)<<", "<<#b<<"="<<(b)<<", "<<#c<<"="<<(c)<<", "<<#d<<"="<<(d)<<"\n"
#define Debug5(a,b,c,d,e)   cout<<#a<<"="<<(a)<<", "<<#b<<"="<<(b)<<", "<<#c<<"="<<(c)<<", "<<#d<<"="<<(d)<<", "<<#e<<"="<<(e)<<"\n"
#define Debug6(a,b,c,d,e,f) cout<<#a<<"="<<(a)<<", "<<#b<<"="<<(b)<<", "<<#c<<"="<<(c)<<", "<<#d<<"="<<(d)<<", "<<#e<<"="<<(e)<<", "<<#f<<"="<<(f)<<"\n"
#define Debug(...) macro_dispatcher(Debug, __VA_ARGS__)(__VA_ARGS__)
#define DA(a,n) cout<<#a<<"=["; printarray(a,n); cout<<"]\n"
#define DAR(a,n,s) cout<<#a<<"["<<s<<"-"<<n-1<<"]=["; printarray(a,n,s); cout<<"]\n"

#define TT1 template<class T>
#define TT1T2 template<class T1, class T2>
#define TT1T2T3 template<class T1, class T2, class T3>
template<class T, size_t N> ostream& operator << (ostream& os, const array<T, N>& v);
TT1T2 ostream& operator << (ostream& os, const pair<T1, T2>& p){ return os <<"("<<p.first<<", "<< p.second<<")"; }
TT1 ostream& operator << (ostream& os, const vector<T>& v){       bool f=1;os<<"[";for(auto& i : v) { if (!f)os << ", ";os<<i;f=0;}return os << "]"; }
template<class T, size_t N> ostream& operator << (ostream& os, const array<T, N>& v) {     bool f=1;os<<"[";for(auto& i : v) { if (!f)os << ", ";os<<i;f=0;}return os << "]"; }
TT1T2 ostream& operator << (ostream& os, const set<T1, T2>&v){    bool f=1;os<<"[";for(auto& i : v) { if (!f)os << ", ";os<<i;f=0;}return os << "]"; }
TT1T2 ostream& operator << (ostream& os, const multiset<T1,T2>&v){bool f=1;os<<"[";for(auto& i : v) { if (!f)os << ", ";os<<i;f=0;}return os << "]"; }
TT1T2T3 ostream& operator << (ostream& os, const map<T1,T2,T3>& v){ bool f = 1; os << "["; for (auto& ii : v) { if (!f)os << ", "; os << "(" << ii.first << " -> " << ii.second << ") "; f = 0; }return os << "]"; }
TT1T2 ostream& operator << (ostream& os, const multimap<T1, T2>& v){ bool f = 1; os << "["; for (auto& ii : v) { if (!f)os << ", "; os << "(" << ii.first << " -> " << ii.second << ") "; f = 0; }return os << "]"; }
TT1T2 ostream& operator << (ostream& os, priority_queue<T1, T2> v) { bool f = 1; os << "["; while (!v.empty()) { auto x = v.top(); v.pop(); if (!f) os << ", "; f = 0; os << x; } return os << "]"; }
TT1T2 void printarray(const T1& a, T2 sz, T2 beg = 0){ for (T2 i = beg; i<sz; i++) cout << a[i] << " "; cout << endl; }

typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> Pi;
typedef pair<ll, ll> Pll;

#define Fi first
#define Se second
#define pb(x) push_back(x)
#define sz(x) (int)x.size()
#define rep(i, n) for(int i=0;i<n;i++)
#define repp(i, n) for(int i=1;i<=n;i++)
#define all(x) x.begin(), x.end()
#define geti1(X) cin >> X
#define geti2(X,Y) cin >> X >> Y
#define geti3(X,Y,Z) cin >> X >> Y >> Z
#define geti4(X,Y,Z,W) cin >> X >> Y >> Z >> W
#define GET_MACRO(_1,_2,_3,_4,NAME,...) NAME
#define geti(...) GET_MACRO(__VA_ARGS__, geti4, geti3, geti2, geti1) (__VA_ARGS__)

#define INF 987654321

#define endl '\n'

const ll mod = 998244353LL;
#define INF 987654321

int N,M;
int p[1050][1050];
int pa[10500], sz1[10500], sz2[10050];
int find(int x){
	if( x == pa[x] ) return x;
	return pa[x] = find(pa[x]);
}

int dp[1000][1000];

void uni(int a, int b){
	a = find(a); b = find(b);
	if( a == b ) return;
	pa[b] = a;
	sz1[a] += sz1[b];
	sz2[a] += sz2[b];
}

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

	//clock_t begin = clock();
	geti(N,M);
	repp(i,2*N){
		pa[i] = i;
		if( i <= N ) sz1[i] = 1;
		else sz2[i] = 1;
	}
	repp(i,N) p[i][i] = 1;
	repp(j,M){
		int a,b; geti(a,b);
		p[a][b] = 1; p[b][a] = 1;
	}

	
	repp(i,N) repp(j,N){
		if( p[i][j] == 0 ){
			uni(i,N+j); uni(N+i,j);
		}
	}

	repp(i,N){
		if( find(i) == find(i+N) ){
			cout << -1 << endl;
			return 0;
		}
	}

		
	vector<int> vv, v;
	repp(i,2*N){
		if( pa[i] == i ) vv.pb(abs(sz1[i]-sz2[i]));
	}	
	sort(all(vv));
	for(int i=0;i<sz(vv);i+=2){
		v.pb(vv[i]);
	}

	dp[0][0] = 1;


	for(int n=1;n<=sz(v);n++){
		int val = v[n-1];
		for(int j=0;j<=N;j++){
		//	dp[n][j] |= dp[n-1][j];
			dp[n][j] |= dp[n-1][j+val];
			dp[n][j] |= dp[n-1][abs(j-val)];
		}
	}
	
	int ans = M;
	for(int j=0;j<=N;j++){
		if( dp[sz(v)][j] == 1 ){
			int a = (N-j)/2; int b = (N+j)/2;
			//Debug(j,a,b);
			ans = min(ans, a*(a-1)/2 + b*(b-1)/2 );
		}
	}
	
	cout << ans << endl;

}










Measured memory_bytes: 8448