/*input
10 39
7 2
7 1
5 6
5 8
9 10
2 8
8 7
3 10
10 1
8 10
2 3
7 4
3 9
4 10
3 4
6 1
6 7
9 5
9 7
6 9
9 4
4 6
7 5
8 3
2 5
9 2
10 7
8 6
8 9
7 3
5 3
4 5
6 3
2 10
5 10
4 2
6 2
8 4
10 6
*/
#include<bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;

#define int long long 
#define double long double
#define f first
#define s second
#define mp make_pair
#define pb push_back

#define RE(i,n) for (int i = 1; i <= n; i++)
#define RED(i,n) for (int i = n; i > 0; i--)
#define REPS(i,n) for(int i = 1; (i*i) <= n; i++)
#define REP(i,n) for (int i = 0; i < (int)n; i++)
#define FOR(i,a,b) for (int i = a; i < b; i++)
#define REPD(i,n) for (int i = n-1; i >= 0; i--)
#define FORD(i,a,b) for (int i = a; i >= b; i--)

#define all(v) v.begin(),v.end()
#define pii pair<int,int>
#define vi vector<int>
#define vvi vector<vi>
#define print(arr) for (auto it = arr.begin(); it != arr.end(); ++it) cout << *it << " "; cout << endl;
#define debug(x) cout << x << endl;
#define debug2(x,y) cout << x << " " << y << endl;
#define debug3(x,y,z) cout << x << " " << y << " " << z << endl;

typedef tree<
int,
null_type,
less<int>,
rb_tree_tag,
tree_order_statistics_node_update>
ordered_set;

const int INF = 1e18+1;
const int MOD = 1e9+7;
const double PI = 3.14159265358979323846264338;

int raise(int a,int n,int m = MOD){
  if(n == 0)return 1;
  if(n == 1)return a;
  int x = 1;
    x *= raise(a,n/2,m);
    x %= m;
    x *= x;
    x %= m;
    if(n%2)x*= a;
    x %= m;
    return x;
}

int floor1(int n,int k){
    if(n%k == 0 || n >= 0)return n/k;
    return (n/k)-1;
}

int ceil1(int n,int k){
    return floor1(n+k-1,k);
}

const int N = 769;
int mat[N][N];
int n,m;
int ans = 0;
// ans = -comp(l)*comp(r) + L*R 
// N-R*R = N*R - R*R
int color[N];
int l[N];
int r[N];
int comp = 0;
bool f = 0;
bool dp[N][N];// can we have sum j till i th index ? 

void bfs(int source){
	queue<int> q;
	q.push(source);
	color[source] = 1;
	int red = 1;
	int blue = 0;
	int edges = 0;
	while(!q.empty()){
		int u = q.front();
		q.pop();
		for(int v = 1;v <= n;v++){
			if(mat[u][v])edges++;
			if(!mat[u][v])continue;
			if(color[v] == color[u])f = 1;
			if(color[v])continue;
			color[v] = (color[u] == 1)?2:1;
			(color[v] == 1)?red++:blue++;
			q.push(v);
		}
	}
	comp++;
	l[comp] = blue;
	r[comp] = red;
}

void solve(){
  	cin >> n >> m;
  	RE(i,n){
  		color[i] = 0;
  		RE(j,n){
          mat[i][j] = 0;
          dp[i][j] = 0;
        }
  	}
  	RE(i,m){
  		int a,b;cin >> a >> b;
  		mat[a][b] = mat[b][a] = 1;
  	}
  	RE(i,n){
  		RE(j,n){
  			if(i != j and !mat[i][j])mat[i][j] = 1;
  			else mat[i][j] = 0;
  		}
  	}
  	RE(i,n){
  		if(!color[i])bfs(i);
  	}
  	if(f){
	  cout << -1;
      return;
  	}
  	int mx = 0;
  	dp[0][0] = 1;
  	for(int i = 1;i <= comp;i++){
  		//cout << l[i] << " " << r[i] << endl;
  		for(int j = 0;j <= n;j++){
  			//dp[i][j] = dp[i-1][j];
  			if(j >= r[i])dp[i][j] |= dp[i-1][j-r[i]];
  			if(j >= l[i])dp[i][j] |= dp[i-1][j-l[i]];
  		}
  	}
  	int cur = INF;
  	int nchoose = n*(n-1);
  	nchoose /= 2;
  	for(int i = 0;i <= n;i++){
  		if(dp[comp][i] == 0)continue;
  		int w = i;
  		int s = n-i;
  		int wow = w*s;
  		cur = min(cur,nchoose-wow);
  	}
  	cout << cur;
}

signed main(){
  ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
  int t = 1;
  //cin >> t;
  while(t--){
    solve();
  }
  return 0;
}


Measured memory_bytes: 4992