#pragma GCC optimize("Ofast","unroll-loops")
#pragma GCC push_options
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#include<bits/stdc++.h>
#include <xmmintrin.h>
#include <immintrin.h>
using namespace::std;
__attribute__((constructor))void init(){cin.tie(0);ios::sync_with_stdio(false);cout<<fixed<<setprecision(15);}
#include<ext/pb_ds/assoc_container.hpp>
#include<ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/priority_queue.hpp>
#include<ext/pb_ds/tag_and_trait.hpp>
// #include <boost/multiprecision/cpp_dec_float.hpp>
// #include <boost/multiprecision/cpp_int.hpp>
// namespace mp = boost::multiprecision;
// typedef mp::number<mp::cpp_dec_float<0>> cdouble;
// typedef mp::cpp_int cint;
template<typename T>using pbds=__gnu_pbds::tree<T,__gnu_pbds::null_type,less<T>,__gnu_pbds::rb_tree_tag,__gnu_pbds::tree_order_statistics_node_update>;
template<typename T>using pbds_map=__gnu_pbds::tree<T,T,less<T>,__gnu_pbds::rb_tree_tag,__gnu_pbds::tree_order_statistics_node_update>;
template<typename T,typename E>using hash_map=__gnu_pbds::gp_hash_table<T,E>;
template<typename T>using pqueue =__gnu_pbds::priority_queue<T, greater<T>,__gnu_pbds::rc_binomial_heap_tag>;
typedef long long lint;
#define INF (1LL<<60)
#define IINF (1<<30)
#define EPS (1e-10)
//#define MOD 1000000007LL
#define MOD 998244353LL
typedef vector<lint> vec;
typedef vector<vector<lint>> mat;
typedef vector<vector<vector<lint>>> mat3;
typedef vector<string> svec;
typedef vector<vector<string>> smat;
template<typename T>inline void numout(T t){bool f=0;for(auto i:t){cout<<(f?" ":"")<<i<INF/2?i:"INF";f=1;}cout<<endl;}
template<typename T>inline void numout2(T t){for(auto i:t)numout(i);}
template<typename T>inline void output(T t){bool f=0;for(auto i:t){cout<<(f?" ":"")<<i;f=1;}cout<<endl;}
template<typename T>inline void output2(T t){for(auto i:t)output(i);}
template<typename T>inline void _output(T t){bool f=0;for(lint i=0;i<t.size();i++){cout<<f?"":" "<<t[i];f=1;}cout<<endl;}
template<typename T>inline void _output2(T t){for(lint i=0;i<t.size();i++)output(t[i]);}
#define rep(i,n) for(lint i=0;i<lint(n);++i)
#define repi(i,a,b) for(lint i=lint(a);i<(lint)(b);++i)
#define rrep(i,n) for(lint i=lint(n)-1;i>=0;--i)
#define rrepi(i,a,b) for(lint i=lint(b)-1;i>=lint(a);--i)
#define irep(i) for(lint i=0;;++i)
#define all(n) begin(n),end(n)
#define dist(a,b,c,d) sqrt(pow(a-c,2)+pow(b-d,2))
inline lint gcd(lint A,lint B){return B?gcd(B,A%B):A;}
inline lint lcm(lint A,lint B){return A/gcd(A,B)*B;}
// inline cint cgcd(cint A,cint B){return B?cgcd(B,A%B):A;}
// inline cint clcm(cint A,cint B){return A/cgcd(A,B)*B;}
inline bool chmin(auto& s,const auto& t){bool res=s>t;s=min(s,t);return res;}
inline bool chmax(auto& s,const auto& t){bool res=s<t;s=max(s,t);return res;}
const vector<lint> dx={1,0,-1,0,1,1,-1,-1};
const vector<lint> dy={0,1,0,-1,1,-1,1,-1};
#define SUM(v) accumulate(all(v),0LL)
auto call=[&](auto f,auto... args){return f(f,args...);};

auto tree_dp(const auto& g,const auto& f,const auto& res_e,int now=0,int par=-1){
    auto res=res_e;
    for(auto e:g[now]){
        res=f(res,tree_dp(g,f,res_e,e,now));
    }
    return res;
}

class UF{
    public:
    vector<int> data;
    int sz;
	public:
    UF(auto sz):sz(sz){data.resize(sz,-1);}
    bool unite(auto x,auto y){
        x=root(x);y=root(y);
        if(x==y)return 0;
        if(data[x]>data[y])swap(x,y);
		data[x]+=data[y];
		data[y]=x;
		sz--;
        return 1;
    }
    inline int root(auto x){return data[x]<0?x:data[x]=root(data[x]);}
    inline bool same(auto x, auto y){return root(x)==root(y);}
    inline int size(){return sz;}
	inline int size(auto x){return -data[root(x)];}
};

int main(){
    lint n,m;
    cin>>n>>m;
    mat v(n,vec(n,1));
    rep(i,n)v[i][i]=0;
    rep(i,m){
        lint s,t;
        cin>>s>>t;
        s--;t--;
        v[s][t]=0;
        v[t][s]=0;
    }
    UF uf(2*n);
    rep(i,n)rep(j,n){
        if(v[i][j]){
            uf.unite(i,n+j);
            uf.unite(i+n,j);
        }
    }
    map<lint,lint>ma;
    vector<pair<lint,lint>> tmp;
    set<lint>used;
    rep(i,n){
        if(uf.same(i,i+n)){
            cout<<-1<<endl;
            return 0;
        }
        ma[uf.root(i)]++;
    }
    rep(i,n){
        if(!used.count(uf.root(i))){
            used.insert(uf.root(i));
            used.insert(uf.root(n+i));
            tmp.emplace_back(ma[uf.root(i)],ma[uf.root(n+i)]);
        }
    }
    bitset<1000>b;
    b[0]=1;
    rep(i,tmp.size()){
        b=b<<tmp[i].first|b<<tmp[i].second;
    }
    lint ans=INF;
    rep(i,n){
        if(b[i]){
            chmin(ans,i*(i-1)/2+(n-i)*(n-i-1)/2);
        }
    }
    cout<<ans<<endl;
}

Measured memory_bytes: 4224