#include <bits/stdc++.h>
using namespace std;
struct fast_ios {
  fast_ios() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(20);
  };
} fast_ios_;
template <typename T>
istream &operator>>(istream &is, vector<T> &v) {
  for (auto &x : v) is >> x;
  return is;
}
template <typename T>
ostream &operator<<(ostream &os, vector<T> &v) {
  for (long long i = 0; i < v.size(); i++) {
    cout << v[i];
    if (i != v.size() - 1) cout << endl;
  };
  return os;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, pair<T1, T2> p) {
  cout << '(' << p.first << ',' << p.second << ')';
  return os;
}
template <typename T>
void Out(T x) {
  cout << x << endl;
}
template <typename T1, typename T2>
void Ans(bool first, T1 y, T2 n) {
  if (first)
    Out(y);
  else
    Out(n);
}
using vec = vector<long long>;
using mat = vector<vec>;
using Pii = pair<long long, long long>;
using PiP = pair<long long, Pii>;
using PPi = pair<Pii, long long>;
using bools = vector<bool>;
using pairs = vector<Pii>;
const long long mod = 1000000007;
struct edge {
  long long to, cost, id;
};
class Graph {
 public:
  long long N;
  vector<vector<edge>> G;
  bools is_black;
  Graph(long long N) : N(N) {
    G = vector<vector<edge>>(N, vector<edge>(0));
    is_black = bools(N, false);
  }
  void add_Directed_edge(long long from, long long to, long long cost = 1,
                         long long id = 0) {
    G[from].push_back(edge({to, cost, id}));
  }
  void add_Undirected_edge(long long v1, long long v2, long long cost = 1,
                           long long id = 0) {
    add_Directed_edge(v1, v2, cost, id);
    add_Directed_edge(v2, v1, cost, id);
  }
  long long dfs(long long v, long long p = -1) {
    long long n = 0;
    for (auto e : G[v])
      if (e.to != p) {
        n += dfs(e.to, v);
      }
    if (!is_black[v]) {
      is_black[v] = true;
      is_black[p] = !is_black[p];
      n++;
    }
    return n;
  }
};
signed main() {
  long long N, M;
  cin >> N >> M;
  assert(M == N - 1);
  if (N % 2) {
    Out(-1);
    return 0;
  }
  Graph G(N);
  for (long long i = (0); i < (M); i++) {
    long long a, b;
    cin >> a >> b;
    a--;
    b--;
    G.add_Undirected_edge(a, b);
  }
  long long ans = G.dfs(0);
  Out(ans);
  return 0;
}
