#include <bits/stdc++.h>
using namespace std;
inline int in() {
  int x;
  scanf("%d", &x);
  return x;
}
inline void priv(vector<int> a) {
  for (int i = 0; i < ((int)(a).size()); ++i)
    printf("%d%c", a[i], i == (int)(a).size() - 1 ? '\n' : ' ');
}
template <typename T>
istream& operator>>(istream& i, vector<T>& v) {
  for (int j = 0; j < ((int)(v).size()); ++j) i >> v[j];
  return i;
}
template <typename T>
string join(vector<T>& v) {
  stringstream s;
  for (int i = 0; i < ((int)(v).size()); ++i) s << ' ' << v[i];
  return s.str().substr(1);
}
template <typename T>
ostream& operator<<(ostream& o, vector<T>& v) {
  if ((int)(v).size()) o << join(v);
  return o;
}
template <typename T1, typename T2>
istream& operator>>(istream& i, pair<T1, T2>& v) {
  return i >> v.first >> v.second;
}
template <typename T1, typename T2>
ostream& operator<<(ostream& o, pair<T1, T2>& v) {
  return o << v.first << "," << v.second;
}
const int MX = 100005, INF = 1001001001;
const long long int LINF = 1e18;
const double eps = 1e-10;
int n, m;
vector<int> to[MX];
long long int ans;
int dfs(int v, int p = -1) {
  int s = 1;
  for (int u : to[v])
    if (u != p) {
      s -= dfs(u, v);
    }
  ans += abs(s);
  return s;
}
int main() {
  scanf("%d%d", &n, &m);
  assert(n == m + 1);
  for (int i = 0; i < (m); ++i) {
    int a, b;
    scanf("%d%d", &a, &b);
    --a;
    --b;
    to[a].push_back(b);
    to[b].push_back(a);
  }
  if (dfs(0)) ans = -1;
  cout << ans << endl;
  return 0;
}
