#include <bits/stdc++.h>
 
using namespace std;
using ll = long long;
// #define int ll
using PII = pair<int, int>;
 
#define FOR(i, a, n) for (ll i = (ll)a; i < (ll)n; ++i)
#define REP(i, n) FOR(i, 0, n)
#define ALL(x) x.begin(), x.end()
 
template<typename T> T &chmin(T &a, const T &b) { return a = min(a, b); }
template<typename T> T &chmax(T &a, const T &b) { return a = max(a, b); }
template<typename T> bool IN(T a, T b, T x) { return a<=x&&x<b; }
template<typename T> T ceil(T a, T b) { return a/b + !!(a%b); }
 
template<typename T> vector<T> make_v(size_t a) { return vector<T>(a); }
template<typename T,typename... Ts>
auto make_v(size_t a,Ts... ts) { 
  return vector<decltype(make_v<T>(ts...))>(a,make_v<T>(ts...));
}
template<typename T,typename V> typename enable_if<is_class<T>::value==0>::type
fill_v(T &t, const V &v) { t=v; }
template<typename T,typename V> typename enable_if<is_class<T>::value!=0>::type
fill_v(T &t, const V &v ) { for(auto &e:t) fill_v(e,v); }
 
template<class S,class T>
ostream &operator <<(ostream& out,const pair<S,T>& a){
  out<<'('<<a.first<<','<<a.second<<')'; return out;
}
template<typename T>
istream& operator >> (istream& is, vector<T>& vec){
  for(T& x: vec) {is >> x;} return is;
}
template<class T>
ostream &operator <<(ostream& out,const vector<T>& a){
  out<<'['; for(T i: a) {out<<i<<',';} out<<']'; return out;
}
 
int dx[] = {0, 1, 0, -1}, dy[] = {1, 0, -1, 0}; // DRUL
const int INF = 1<<30;
const ll LLINF = 1LL<<40;
const ll MOD = 1000000007;

struct dice {
  mt19937 mt;
  dice() {
    random_device rd;
    mt = mt19937(rd());
  }
  int operator()(int x) { return this->operator()(0, x - 1); }
  int operator()(int x, int y) {
    uniform_int_distribution<int> dist(x, y);
    return dist(mt);
  }
} rnd;

class UnionFind {
public:
  vector<int> par, s;
  UnionFind(int n) { init(n); }
  void init(int n) { 
    par.resize(n); s.assign(n, 1);
    iota(ALL(par), 0);
  }
  int find(int x) {
    if(par[x] == x) return x;
    return par[x] = find(par[x]);
  }
  void unite(int x, int y) {
    x = find(x);
    y = find(y);
    if(x == y) return;
    if(s[x] < s[y]) par[x] = y, s[y] = s[x] + s[y];
    else par[y] = x, s[x] = s[x] + s[y];
  }
  bool same(int x, int y) { return find(x) == find(y); }
  int size(int x) { return s[find(x)]; }
};

signed main(void)
{
  cin.tie(0);
  ios::sync_with_stdio(false);

  ll n, m;
  cin >> n >> m;
  auto a = make_v<ll>(n, n);
  REP(i, m) {
    ll u, v;
    cin >> u >> v;
    u--, v--;
    a[u][v] = a[v][u] = true;
  }

  // REP(i, n) cout << a[i] << endl;
  // cout << endl;

  REP(t, 100) {
    auto b = a;
    UnionFind uf(n);
    REP(i, n) REP(j, n) if(a[i][j]) uf.unite(i, j);
    while(uf.size(0) < n) {
      ll u = rnd(n), v = rnd(n);
      if(!uf.same(u, v)) {
        b[u][v] = b[v][u] = true;
        uf.unite(u, v);
      }
    }

    vector<ll> type(n, -1);
    function<void(ll)> divide = [&](ll v) {
      REP(i, n) if(!b[v][i] && type[i] == -1) {
        type[i] = type[v];
        divide(i);
      }
    };
    ll k = 0;
    REP(i, n) if(type[i] == -1) {
      type[i] = k++;
      divide(i);
    }
    // cout << "type " << type << endl;

    vector<ll> deg(n);
    REP(i, n) REP(j, n) if(b[i][j]) deg[i]++;
    // cout << "deg " << deg << endl;
    vector<ll> odd(k);
    REP(i, n) if(deg[i] % 2) odd[type[i]]++;
    // cout << "odd " << odd << endl;
    bool flag = true;
    REP(i, k) if(odd[i]%2) {
      flag = false;
      break;
    }
    if(!flag) continue;

    vector<bool> used(n), isodd(n);
    REP(i, n) isodd[i] = deg[i] % 2;
    auto c = make_v<ll>(n, n);

    function<bool(ll)> dfs = [&](ll v) {
      if(used[v]) return false;
      used[v] = true;
      if(isodd[v]) { 
        // cout << v << " is odd" << endl;
        isodd[v] = false;
        return true; 
      }
      REP(i, n) if(!b[v][i] && dfs(i)) {
        c[v][i] ^= 1;
        c[i][v] ^= 1;
        return true;
      }
      return false;
    };

    REP(i, n) if(isodd[i]) {
      isodd[i] = false;
      used.assign(n, false);
      dfs(i);
    }

    // cout << deg << endl << endl;
    // REP(i, n) cout << b[i] << endl;
    // cout << endl;
    // REP(i, n) cout << c[i] << endl;

    vector<PII> edge;
    REP(i, n) FOR(j, i+1, n) {
      if(!a[i][j] && b[i][j]) edge.push_back({i, j});
      else if(c[i][j]) edge.push_back({i, j});
    }
    cout << edge.size() << endl;
    for(auto e: edge) cout << e.first+1 << " " << e.second+1 << endl;
    return 0;
  }
  cout << -1 << endl;

  return 0;
}



Measured memory_bytes: 3340