#include <bits/stdc++.h>
using namespace std;
template <typename T>
bool get_int(T &x) {
  char t = getchar();
  bool neg = false;
  x = 0;
  for (; (t > '9' || t < '0') && t != '-' && t != EOF; t = getchar())
    ;
  if (t == '-') neg = true, t = getchar();
  if (t == EOF) return false;
  for (; t <= '9' && t >= '0'; t = getchar()) x = x * 10 + t - '0';
  if (neg) x = -x;
  return true;
}
template <typename T>
void print_int(T x) {
  if (x < 0) putchar('-'), x = -x;
  short a[20] = {}, sz = 0;
  while (x > 0) a[sz++] = x % 10, x /= 10;
  if (sz == 0) putchar('0');
  for (int i = sz - 1; i >= 0; i--) putchar('0' + a[i]);
}
const int inf = 0x3f3f3f3f;
const long long Linf = 1ll << 61;
const double pi = acos(-1.0);
vector<int> g[100111];
int n, m, sum[100111];
long long ans;
void dfs(int x, int f = 0, int c = 0) {
  if (c)
    sum[x]++;
  else
    sum[x]--;
  for (auto v : g[x])
    if (v != f) {
      dfs(v, x, c ^ 1);
      sum[x] += sum[v];
      ans += abs(sum[v]);
    }
}
int main() {
  (get_int(n) && get_int(m));
  for (int i = 1, u, v; i <= m; i++) {
    (get_int(u) && get_int(v));
    g[u].push_back(v);
    g[v].push_back(u);
  }
  if (n == m) return 0;
  dfs(1);
  if (sum[1]) ans = -1;
  print_int(ans), puts("");
  return 0;
}
