//http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=2370
#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef pair<int,int> pint;
typedef pair<long,long> plong;
typedef vector<int> vint;
typedef vector<long> vlong;

const double pi=M_PI;
const int INF10 = 1000000001;
const ll INF15 = 1e15 +1;
const long long INF18 = 1e18 + 1;
const int mod = 1000000007;
//const int mod = 998244353;
const double EPS=0.00001;

#define rep(i, n) for (int i = 0; i < (ll)(n); i++)
#define rep1(i, n) for (int i = 1; i <= (ll)(n); i++)
#define rep2(i,start,end) for(int i=(ll)start;i<=(ll)end;i++)
#define vrep(i, n) for(int i=(ll)n-1;i>=0;i--)
#define vrep1(i, n) for(int i=(ll)n;i>0;i--)
#define all(n) n.begin(),n.end()
#define pb push_back
#define debug(x) cerr << #x <<": " << x << '\n'

struct edge{
    edge(int i,long c=1){
        to=i;
        cost=c;
    }
    int to;
    long cost;
};

//bを何回足せばaを超えるか(O(a/b))
//a+b-1/bとすればよし

//2進数表示したときの最高桁(O(log n))
int bi_max(long n){
    int m = 0;
    for (m; (1 << m) <= n; m++);
    m = m - 1;
    return m;
}
//bi_eに二進数表示したやつを代入(O(log^2 n))
//bitset<N> a(n)でnの二進数表示が得られて、a[i]=0or1でi番目のfragが立ってるかわかる
//x^n mod m (nが負の時は０)(O(log n))
long myPow(long x, long n, long m=mod){
    if (n < 0)
        return 0;
    if (n == 0)
        return 1;
    if (n % 2 == 0)
        return myPow(x * x % m, n / 2, m);
    else
        return x * myPow(x, n - 1, m) % m;
}
// auto mod int
// https://youtu.be/L8grWxBlIZ4?t=9858
// https://youtu.be/ERZuLAxZffQ?t=4807 : optimize
// https://youtu.be/8uowVvQ_-Mo?t=1329 : division
struct mint{
    ll x; // typedef long long ll;
    mint(ll x = 0) : x((x % mod + mod) % mod) {}
    mint operator-() const { return mint(-x); }
    mint &operator+=(const mint a)
    {
        if ((x += a.x) >= mod)
            x -= mod;
        return *this;
    }
    mint &operator-=(const mint a)
    {
        if ((x += mod - a.x) >= mod)
            x -= mod;
        return *this;
    }
    mint &operator*=(const mint a)
    {
        (x *= a.x) %= mod;
        return *this;
    }
    mint operator+(const mint a) const { return mint(*this) += a; }
    mint operator-(const mint a) const { return mint(*this) -= a; }
    mint operator*(const mint a) const { return mint(*this) *= a; }
    mint pow(ll t) const
    {
        if (!t)
            return 1;
        mint a = pow(t >> 1);
        a *= a;
        if (t & 1)
            a *= *this;
        return a;
    }

    // for prime mod
    mint inv() const { return pow(mod - 2); }
    mint &operator/=(const mint a) { return *this *= a.inv(); }
    mint operator/(const mint a) const { return mint(*this) /= a; }
    bool operator!=(const int a) { return this->x!=a; }
     bool operator==(const int a) { return this->x==a; }
};
istream &operator>>(istream &is, const mint &a) { return is >> a.x; }
ostream &operator<<(ostream &os, const mint &a) { return os << a.x; }
// combination mod prime
// https://www.youtube.com/watch?v=8uowVvQ_-Mo&feature=youtu.be&t=1619
struct combination{
    vector<mint> fact, ifact;
    combination(int n) : fact(n + 1), ifact(n + 1)
    {
        assert(n < mod);
        fact[0] = 1;
        for (int i = 1; i <= n; ++i)
            fact[i] = fact[i - 1] * i;
        ifact[n] = fact[n].inv();
        for (int i = n; i >= 1; --i)
            ifact[i - 1] = ifact[i] * i;
    }
    mint operator()(int n, int k)
    {
        if (k < 0 || k > n)
            return 0;
        return fact[n] * ifact[k] * ifact[n - k];
    }
};
template<class T>
void maxin (T &a,T b){a=max(a,b);}
template<class T>
void minin (T &a,T b){a=min(a,b);}
template<class M,class N> 
constexpr common_type_t<M,N> gcd(M a,N b){
    a=abs(a);b=abs(b);
    if(a < b) return gcd(b, a);
    M r;
    while ((r=a%b)) {
        a = b;
        b = r;
    }
    return b;
}
template<class M,class N>
constexpr common_type_t<M,N> lcm(M a,N b){
    return a*b/gcd(a,b);
}

const int N_MAX=100005;
ll n,m,col[N_MAX];
vector<int> v[N_MAX];
int dp[N_MAX],nd[N_MAX];

ll solve(){
    ll ano=0;
    vector<pair<ll,ll>> res;
    rep(i,n){
        if(col[i]!=0)continue;
        if(v[i].size()==0){
            ano++;
            continue;
        }
        ll blo=0,whi=0;
        queue<int> que;
        que.push(i);
        col[i]=1;
        while(que.size()){
            int now=que.front();que.pop();
            col[now]==1?blo++:whi++;
            for(auto w:v[now]){
                if(col[w]==0){
                    col[w]=-col[now];
                    que.push(w);
                }else if(col[w]==col[now])return -1;
            }
        }
        res.pb({max(blo,whi),min(blo,whi)});
    }
    ll blo=0,whi=0;
    bitset<N_MAX> a,b;a[0]=1;
    for(auto w:res){
        rep(i,(1+(n+1)/2)-w.first){
            if(a[i])b[i+w.first]=1;
        }
        rep(i,(1+(n+1)/2)-w.second){
            if(a[i])b[i+w.second]=1;
        }
        rep(i,(1+(n+1)/2)){
            a[i]=b[i];
        }
        b.reset();
    }
    vrep(i,1+(n+1)/2){
        if(a[i]){
            blo=i;
            break;
        }
    }
    whi=n-blo-ano;
    /*debug(blo);
    debug(whi);
    debug(ano);*/
    if((n+1)/2<max(blo,whi)){
        blo>whi?whi+=ano:blo+=ano;
        return whi*blo-m;
    }else{
        return ((n+1)/2)*(n-((n+1)/2))-m;
    }
}

int main(){
    int x,y;
    //入力
    cin>>n>>m;
    rep(i,m){
        cin>>x>>y;
        v[x-1].pb(y-1);
        v[y-1].pb(x-1);
    }
    //処理
    auto ans=solve();
    //出力
    cout << ans <<endl;
}


Measured memory_bytes: 10004