#include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
typedef unsigned int uint;
typedef unsigned char uchar;
typedef unsigned long long ull;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;

#define REP(i,x) for(int i=0;i<(int)(x);i++)
#define REPS(i,x) for(int i=1;i<=(int)(x);i++)
#define RREP(i,x) for(int i=((int)(x)-1);i>=0;i--)
#define RREPS(i,x) for(int i=((int)(x));i>0;i--)
#define FOR(i,c) for(__typeof((c).begin())i=(c).begin();i!=(c).end();i++)
#define RFOR(i,c) for(__typeof((c).rbegin())i=(c).rbegin();i!=(c).rend();i++)
#define ALL(container) (container).begin(), (container).end()
#define RALL(container) (container).rbegin(), (container).rend()
#define SZ(container) ((int)container.size())
#define mp(a,b) make_pair(a, b)
#define pb push_back
#define eb emplace_back
#define UNIQUE(v) v.erase( unique(v.begin(), v.end()), v.end() );

template<class T> bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a=b; return 1; } return 0; }
template<class T> ostream& operator<<(ostream &os, const vector<T> &t) {
    os<<"["; FOR(it,t) {if(it!=t.begin()) os<<","; os<<*it;} os<<"]"; return os;
}
template<class T> ostream& operator<<(ostream &os, const set<T> &t) {
    os<<"{"; FOR(it,t) {if(it!=t.begin()) os<<","; os<<*it;} os<<"}"; return os;
}
template<class S, class T> ostream& operator<<(ostream &os, const pair<S,T> &t) { return os<<"("<<t.first<<","<<t.second<<")";}
template<class S, class T> pair<S,T> operator+(const pair<S,T> &s, const pair<S,T> &t){ return pair<S,T>(s.first+t.first, s.second+t.second);}
template<class S, class T> pair<S,T> operator-(const pair<S,T> &s, const pair<S,T> &t){ return pair<S,T>(s.first-t.first, s.second-t.second);}

const int INF = 1<<28;
const double EPS = 1e-8;
const int MOD = 1000000007;


int T, n, m;

bool dfs(vector<vi> &g, vector<int> &color, int u, int c){
    color[u] = c;
    for(int v : g[u]){
        if(color[v] == c) return false;
        if(!color[v] && !dfs(g, color, v, -c)) return false;
    }
    return true;
}

vector<pii> isBipartite(vector<vi> &g){
    int n = g.size();
    vector<int> color(n);
    vector<pii> counts;
    REP(i, n) {
        if(!color[i]) {
            int p = count(ALL(color), 1);
            int q = count(ALL(color), -1);
            if (!dfs(g, color, i, 1)) return vector<pii>();
            counts.emplace_back(count(ALL(color), 1) - p, count(ALL(color), -1) - q);
        }
    }
    return counts;
}

int main(int argc, char *argv[]){
    ios::sync_with_stdio(false);
    cin >> n >> m;
    vector<vi> gl(n, vi(n));
    REP(i, m) {
        int u, v;
        cin >> u >> v; u--; v--;
        gl[u][v] = gl[v][u] = 1;
    }
    vector<vi> g(n);
    REP(i, n) {
        REP(j, i) {
            if (!gl[i][j]) {
                g[i].push_back(j);
                g[j].push_back(i);
            }
        }
    }
    vector<pii> cnt = isBipartite(g);
    if (cnt.empty()) cout << -1 << endl;
    else {
        vi dp(n + 1);
        dp[0] = 1;
        for (auto it : cnt){
            vi nxt(n + 1);
            RREP(i, n + 1){
                if (i + it.first <= n) {
                    nxt[i + it.first] |= dp[i];
                }
                if (i + it.second <= n) {
                    nxt[i + it.second] |= dp[i];
                }
            }
            swap(nxt, dp);
        }
        int ans = MOD;
        REP(i, n + 1) {
            if (dp[i]) ans = min(ans, i * (i - 1) / 2 + (n - i) * (n - i - 1) / 2);
        }
        cout << ans << endl;
    }
    return 0;
}


Measured memory_bytes: 4992