#pragma comment(linker, "/stack:200000000")
#pragma GCC optimize("unroll-loops")

#include <bits/stdc++.h>
// #include <ext/pb_ds/assoc_container.hpp>
// #include <ext/pb_ds/tree_policy.hpp>
// #include <ext/pb_ds/detail/standard_policies.hpp>

using namespace std;
// using namespace __gnu_pbds;
// using namespace __gnu_cxx;

typedef long long ll;
typedef unsigned int ul;
typedef unsigned long long ull;
typedef vector <int> vi;
typedef map<int, vector<int> > mvii;
typedef map<int, int> mii;
typedef queue <int> qi;
typedef vector <string> vs;
typedef pair <int, int> pii;
typedef vector<pii > vpii;

// Order Statistic Tree

/* Special functions: 

		find_by_order(k) --> returns iterator to the kth largest element counting from 0
		order_of_key(val) --> returns the number of items in a set that are strictly smaller than our item
*/

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

#define MP make_pair
#define SORT(a) sort (a.begin(), a.end())
#define REVERSE(a) reverse (a.begin(), a.end())
#define ALL(a) a.begin(), a.end()
#define PI acos(-1)
#define ms(x,y) memset (x, y, sizeof (x))
#define inf 1e9
#define INF 1e16
#define pb push_back
#define MAX 100005
#define debug(a,b) cout<<a<<": "<<b<<endl
#define Debug cout<<"Reached here"<<endl
#define prnt(a) cout<<a<<"\n"
#define mod 1000000007LL
#define FOR(i,a,b) for (int i=(a); i<(b); i++)
#define FORr(i,a,b) for (int i=(a); i>=(b); i--)
#define itrALL(c,itr) for(__typeof((c).begin()) itr=(c).begin();itr!=(c).end();itr++)
#define lc ((node)<<1)
#define rc ((node)<<1|1)
#define VecPrnt(v) FOR(J,0,v.size()) cout<<v[J]<<" "; cout<<endl
#define endl "\n"
#define PrintPair(x) cout<<x.first<<" "<<x.second<<endl
#define EPS 1e-9
#define ArrPrint(a,st,en) for(int J=st; J<=en; J++) cout<<a[J]<<" "; cout<<endl;

/* Direction Array */

// int fx[]={1,-1,0,0};
// int fy[]={0,0,1,-1};
// int fx[]={0,0,1,-1,-1,1,-1,1};
// int fy[]={-1,1,0,0,1,1,-1,-1};

template <class T> inline T bigmod(T p,T e,T M)
{
    ll ret = 1;
    for(; e > 0; e >>= 1)
    {
        if(e & 1) ret = (ret * p) % M;
        p = (p * p) % M;
    } return (T)ret;
}

template <class T> inline T gcd(T a,T b){if(b==0)return a;return gcd(b,a%b);}
template <class T> inline T modinverse(T a,T M){return bigmod(a,M-2,M);}
template <class T> inline T lcm(T a,T b) {a=abs(a);b=abs(b); return (a/gcd(a,b))*b;}
template <class T, class X> inline bool getbit(T a, X i) { T t=1; return ((a&(t<<i))>0);}
template <class T, class X> inline T setbit(T a, X i) { T t=1;return (a|(t<<i)); }
template <class T, class X> inline T resetbit(T a, X i) { T t=1;return (a&(~(t<<i)));}

inline ll getnum()
{
    char c = getchar();
    ll num,sign=1;
    for(;c<'0'||c>'9';c=getchar())if(c=='-')sign=-1;
    for(num=0;c>='0'&&c<='9';)
    {
        c-='0';
        num = num*10+c;
        c=getchar();
    }
    return num*sign;
}

inline ll power(ll a, ll b)
{
	ll multiply=1;
	FOR(i,0,b)
	{
		multiply*=a;
	}
	return multiply;
}

/****** END OF HEADER ******/

const int N=705;
int n, m, comp=0, color[N], cnt[N][3];
bool in[N][N], flag;
vi graph[N];
int dp[N][N];

void dfs(int u, int col)
{
	color[u]=col;
	cnt[comp][col]++;

	for(auto v: graph[u])
	{
		if(color[v]!=-1)
		{
			if(color[v]==col)
			{
				flag=false;
				return;
			}
			else continue;
		}
		dfs(v,3-col);
	}
}

bool go(int curr, int rem)
{
	if(curr>comp) return rem==0;
	if(dp[curr][rem]!=-1) return dp[curr][rem];

	bool ret=false;

	if(rem>=cnt[curr][1]) ret|=go(curr+1,rem-cnt[curr][1]);
	if(rem>=cnt[curr][2]) ret|=go(curr+1,rem-cnt[curr][2]);

	return dp[curr][rem]=ret;
}

void solve()
{
	ms(color,-1);

	FOR(i,1,n+1)
	{
		if(color[i]==-1)
		{
			comp++;
			flag=true;
			dfs(i,1);

			if(!flag)
			{
				prnt(-1);
				return;
			}
		}
	}

	// debug("comp",comp);

	ll ans=INF;
	ms(dp,-1);	

	FOR(i,0,n+1)
	{
		if(go(1,i))
		{
			ans=min(ans,((ll)i*(i-1))/2+((ll)(n-i)*(n-i-1))/2);
		}
	}

	prnt(ans);
}

int main()
{
    // ios_base::sync_with_stdio(0);
    // cin.tie(NULL); cout.tie(NULL);
    // freopen("in.txt","r",stdin);

    int test, cases=1;

	cin>>n>>m;

	int u, v;

	FOR(i,1,m+1)
	{
		cin>>u>>v;
		in[u][v]=true;
		in[v][u]=true;
	}

	FOR(i,1,n+1)
	{
		FOR(j,i+1,n+1)
		{
			if(!in[i][j])
			{
				graph[i].pb(j);
				graph[j].pb(i);
			}
		}
	}

	solve();

    return 0;
}



Measured memory_bytes: 3072