#include <iostream>
#include <cstdio>
#include <cassert>
#include <cstring>
#include <vector>
#include <valarray>
#include <array>
#include <queue>
#include <set>
#include <unordered_set>
#include <map>
#include <unordered_map>
#include <algorithm>
#include <cmath>
#include <complex>
#include <random>
#include <bitset>

using namespace std;
typedef long long ll;

const int MN = 100100;
typedef pair<int, int> P;

vector<int> g[MN];
bool used[MN];
bool color[MN];
int c0, c1;
bool err;
void dfs(int p, bool c) {
    used[p] = true;
    color[p] = c;
    if (!c) c0++;
    else c1++;
    for (int d: g[p]) {
        if (used[d]) {
            if (color[p] == color[d]) err = true;
            continue;
        }
        dfs(d, !c);
    }
}

bitset<MN> b;
ll solve() {
    int v, e;
    cin >> v >> e;
    for (int i = 0; i < e; i++) {
        int a, b;
        cin >> a >> b; a--; b--;
        g[a].push_back(b);
        g[b].push_back(a);
    }
    b[0] = true;
    for (int i = 0; i < v; i++) {
        if (used[i]) continue;
        c0 = c1 = 0;
        dfs(i, 0);
        if (err) return -1;
        b = (b<<c0) | (b<<c1);
    }
    ll sm = -1;
    for (int i = 0; i < v; i++) {
        if (!b[i]) continue;
        sm = max(sm, (ll)(v-i)*i-e);
    }
    return sm;
}


int main() {
    cout << solve() << endl;
    return 0;
}