#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#define int long long

#define all(X) (X).begin(), (X).end()
#define reset(X) memset(X, 0, sizeof(X))
#define mp(a, b) make_pair(a, b)
#define pb push_back
#define endl '\n'
#define fi first
#define se second

using namespace __gnu_pbds;
using namespace std;
const int mod = 1e9 + 7;
// const int mod=998244353;
//const int mod=1e9+9;
//const int INF=4e18+10;
const int INF = 4e18 + 10;
template <typename T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
template <typename T>
using ordered_multiset = tree<T, null_type, less_equal<T>, rb_tree_tag, tree_order_statistics_node_update>;

#define read(n) \
int n;      \
cin >> n;
#define readv(x, n)   \
vector<int> x(n); \
for (auto &i : x) \
    cin >> i;

typedef vector<int> vi;
typedef vector<vector<int>> vvi;
typedef pair<int, int> pii;

template <typename T1, typename T2>
void set_min(T1 &a, T2 b)
{
    a = min(a, (T1)b);
}

int multiply(int a, int b, int in_mod) { return (a * b) % in_mod; }
int mult_identity(int a) { return 1; }


auto power(auto a, int b, int in_mod)
{
    auto prod = mult_identity(a);
    auto mult = a % in_mod;
    while (b != 0)
    {
        if (b % 2)
        {
            prod = multiply(prod, mult, in_mod);
        }
        mult = multiply(mult, mult, in_mod);
        b /= 2;
    }
    return prod;
}
int mod_inv(int q, int in_mod)
{

    return power(q, in_mod - 2, in_mod);
}


int S(int x)
{
    int sm = 0;
    while(x)
    {
        sm+=x%10;
        x/=10;
    }
    return sm;
}
void solve()

{
    int n;
    cin>>n;
    int m;
    cin>>m;
    vector<vector<int>> adj(n+1, vector<int>(n+1, 1));
    for(int i=0;i<m;i++)
    {
        int a, b;
        cin>>a>>b;
        adj[a][b]=0;
        adj[b][a]=0;

    }

    vector<int> real_adj[n+1];
    for(int i =1;i<=n;i++)
    {
        for(int j= 1;j<=n;j++)
        {
            if(adj[i][j])
                if(i!=j)
                real_adj[i].pb(j);
        }
    }


    //check bipartite

    vector<int> visited(n+1);
    bool flag=false;
    vector<pair<int, int>> sz;
    int ind  = -1; 
    int value =0;
    int val2 = 0;
    function<void(int, int)> dfs = [&](int x, int val)
    {

        visited[x] = val;
        if(val == 1)
            value++;
        if(val == 2)
            val2++;
        for(auto i:real_adj[x])
            if(!visited[i])
            {
                dfs(i, 3-val);
            }
            else
                flag = flag ||(3-val != visited[i]);


    };

    for(int i=1 ;i<=n;i++)
    {
        if(!visited[i])
        {
            ind++;
            value = 0;
            val2=0;
            dfs(i, 1);
            sz.pb({value, val2});
        }
    }
    if(flag)
        cout<<-1<<endl;
    else
    {

        //build the dp

        bitset<800+1> p;
        p[0] = true;
        for(auto i:sz)
        {
            auto orig_p = p;
            p = orig_p<<i.fi;
            p |= orig_p<<(i.se);
        }

        int mn = INF;
        for(int i= 0;i<=800;i++)
            if(p[i])
                mn= min(mn, i*(i-1)/2 + (n-i)*(n-i-1)/2);
        cout<<mn<<endl;
    }



}


signed main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cerr.tie(NULL);
#ifndef ONLINE_JUDGE
    if (fopen("INPUT.txt", "r"))
    {
        freopen("INPUT.txt", "r", stdin);
        freopen("OUTPUT.txt", "w", stdout);
    }
#endif
    auto clk = clock();
    mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
    // -------------------------------------Code starts here---------------------------------------------------------------------

    int t = 1;
    // cin >> t;

    for (int test = 1; test <= t; test++)
    {
        // cout<<"Case #"<<test<<":\n";
        //cout<<"Trip #"<<test<<": ";
        // cout<<endl;
        solve();
        // solveB();

        // cout<<endl;
    }

    // -------------------------------------Code ends here------------------------------------------------------------------

    clk = clock() - clk;
    // cerr << fixed << setprecision(6) << "Time: " << ((long double)clk) / CLOCKS_PER_SEC << "\n";
    return 0;
}


Measured memory_bytes: 12896