#include <bits/stdc++.h>

using namespace std;

using i64 = int64_t;

const i64 MOD = 1e9+7;

const i64 INF = 1e18+7;

// pythonのrangeのような範囲for文用のclass for(auto& i : Range<>(10)) のように書く
template <typename T = i64>
struct Range{
    struct iterator{
        T value;
        const T step, last;
        const T& operator*(){return value;}
        iterator(T value, T step, T last) :
            value(value),
            step(step),
            last(last)
        {
        }
        iterator operator++(){value = step < static_cast<T>(0) ? max(value + step, last) : min(value + step, last); return *this;}
        bool operator!=(const iterator& x){return value != x.value;}
    };
    const T start, last, step;

    Range(const T start, const T last, const T step = static_cast<T>(1)) :
        start(start),
        last(last),
        step(step)
    {
    }

    Range(const T last) :
        start(0),
        last(last),
        step(1)
    {
    }

    iterator begin(){return iterator(start, step, last);}
    iterator end(){return iterator(last, step, last);}
};

// lambda式を用いた再帰
template <typename F>
struct FixPoint{
    const F _f;
    FixPoint(F&& f) : _f(forward<F>(f)){}

    template<typename... Types>
    decltype(auto) operator()(Types&&... args) const{
        return _f(*this, forward<Types>(args)...);
    }
};

template <typename F>
static decltype(auto) makeRec(F&& f){
    return FixPoint<F>(forward<F>(f));
}

// 多次元vectorの一斉初期化 makeVector<i64, 0>(a, b, ...)のように書く
template <typename T, T Value = T()>
vector<T> makeVector(size_t x){
    return vector<T>(x, T(Value));
}

template <typename T, T Value = T(), typename... Types>
auto makeVector(size_t x, Types... args){
    return vector<decltype(makeVector<T, Value>(args...))>(x, makeVector<T, Value>(args...));
}

// 最大値を更新し、更新できた時にはtrueを返す
template <typename T = i64>
bool chmax(T& a, T b){
    if(a < b){
        a = b;
        return true;
    }
    return false;
}

// 同様に最小値を更新する
template <typename T = i64>
bool chmin(T& a, T b){
    if(a > b){
        a = b;
        return true;
    }
    return false;
}

// 行数と変数名、値をclogに表示するデバッグ用print
#define dump(x) fprintf(stderr, "line =%4d, name =%7s , ", __LINE__, #x); clog << "value = " << x << endl;

// 同様の配列向けデバッグ用print
#define vecdump(x) fprintf(stderr, "line =%4d, name =%7s\n", __LINE__, #x); _dump_macro(x);

void _dump(int, string& x){
    clog << x << " ";
}

template <typename T>
void _dump(bool, T& x){
    clog << x << " ";
}

template <typename T, typename U = typename T::iterator>
void _dump(int, T& x){

    for(auto& elm : x)
        _dump(0, elm);

    clog << endl;
}

template <typename T>
void _dump_macro(T& x){
    _dump(0, x);
}

// input用の関数群
void _input(int, string& x){
    cin >> x;
}

template <typename T>
void _input(bool, T& x){
    cin >> x;
}

template <typename T, typename U = typename T::iterator>
void _input(int, T& x){

    for(auto& elm : x)
        _input(0, elm);
}

template <typename T>
void input_single(T& x){
    _input(0, x);
}

auto input(){}

template <typename T, typename... Types>
void input(T& value, Types&&... args){
    input_single(value);
    input(forward<Types>(args)...);
};

void _pararell_input(size_t){}

template <typename T, typename... Types>
void _pararell_input(size_t index, T& value, Types&&... args){
    input(value[index]);
    _pararell_input(index, forward<Types>(args)...);
}

template <typename... Types>
void pararell_input(size_t count, Types&&... args){
    for(const auto& i : Range<>(count))
        _pararell_input(i, forward<Types>(args)...);
}

struct UnionFind{
    vector<int> par;
    UnionFind(int n) : par(n, -1){}
    int Find(int x){return par[x] < 0 ? x : Find(par[x]);}
    bool Unite(int x, int y){
        x = Find(x), y = Find(y);
        if(x == y)
            return false;
        if(par[x] > par[y])
            swap(x, y);
        par[x] += par[y];
        par[y] = x;
        return true;
    }
};

signed main(){

    cin.tie(0);
    ios::sync_with_stdio(false);

    int n, m;
    input(n, m);
    vector<int> a(m), b(m);
    pararell_input(m, a, b);
    auto v = makeVector<i64>(n, n);
    for(const auto& i : Range<>(m)){
        --a[i], --b[i];
        v[a[i]][b[i]] = 1;
        v[b[i]][a[i]] = 1;
    }

    auto inv = makeVector<i64>(n, 0);
    for(const auto& i : Range<>(n))
        for(const auto& j : Range<>(n))
            if(i != j && !v[i][j])
                inv[i].emplace_back(j);

    UnionFind u(n);

    vector<int> col(n, -1);

    for(const auto& i : Range<>(n))
        for(auto& j : inv[i])
            u.Unite(i, j);

    auto f = makeRec([&](auto&& f, int pos) -> bool{
        for(auto& ed : inv[pos]){
            if(col[ed] == -1){
                col[ed] = col[pos] ^ 1;
                if(!f(ed))
                    return false;
            }else if(col[pos] == col[ed]){
                return false;
            }
        }
        return true;
    });

    for(const auto& i : Range<>(n)){
        if(col[i] == -1){
            col[i] = 0;
            if(!f(i)){
                cout << -1 << endl;
                return 0;
            }
        }
    }

    auto c = makeVector<i64>(n, 2);
    for(const auto& i : Range<>(n)){
        ++c[u.Find(i)][col[i]];
    }

    bitset<707> bs;
    bs.set(0);
    for(const auto& i : Range<>(n)){
        bs = (bs << c[i][0]) | (bs << c[i][1]);
    }

    i64 ans = INF;
    for(const auto& i : Range<>(707))
        if(bs[i]){
            int j = n - i;
            chmin(ans, (i * (i - 1)) / 2 + (j * (j - 1)) / 2);
        }

    cout << ans << endl;


}


Measured memory_bytes: 9728