#include <bits/stdc++.h>
using namespace std;
const int MAXN = 100010;
template <typename T>
inline T read() {
  T x(0), f(1);
  char ch = getchar();
  for (; !isdigit(ch); ch = getchar())
    if (ch == '-') f = -1;
  for (; isdigit(ch); ch = getchar()) x = (x * 10) + (ch ^ 48);
  return x * f;
}
int n, m;
int e, st[MAXN], to[MAXN << 1];
int nxt[MAXN << 1];
int dep[MAXN], A, B;
bool vis[MAXN], cycle;
inline void Add(int u, int v) {
  to[++e] = v, nxt[e] = st[u];
  st[u] = e;
}
namespace Tree {
int ans, ans1, ans2, size[MAXN];
int dfs(int u, int fa, int dep) {
  int res = dep & 1 ? 1 : 0;
  if (dep & 1)
    ans1++;
  else
    ans2++;
  size[u] = 1;
  for (int i = st[u]; i; i = nxt[i]) {
    if (to[i] == fa) continue;
    res += dfs(to[i], u, dep + 1);
    size[u] += size[to[i]];
  }
  if (u != 1) ans += abs(size[u] - (res << 1));
  return res;
}
inline void solve() {
  for (int i = 1; i < n; i++) {
    int u = read<int>(), v = read<int>();
    Add(u, v), Add(v, u);
  }
  dfs(1, 1, 1);
  if (ans1 == ans2)
    printf("%d\n", ans);
  else
    printf("-1\n");
}
}  // namespace Tree
namespace Odd {
int n1, n2, val[MAXN];
int size[MAXN], ans;
bool L;
int dfs(int u, int fa) {
  int res = val[u];
  size[u] = (dep[u] & 1) != L;
  for (int i = st[u]; i; i = nxt[i]) {
    int v = to[i];
    if (v == fa) continue;
    if (v == A && u == B) continue;
    if (v == B && u == A) continue;
    res += dfs(v, u);
    size[u] += size[v];
  }
  ans += abs(size[u] - res);
  return res;
}
inline void solve() {
  L = dep[A] & 1;
  for (int i = 1; i <= n; i++)
    if ((dep[i] & 1) == L) {
      val[i]++;
      n1++;
    } else
      n2++;
  if ((abs(n2 - n1)) & 1) {
    printf("-1\n");
    return;
  }
  val[A] += (n2 - n1) >> 1, val[B] += (n2 - n1) >> 1;
  dfs(1, 0);
  printf("%d\n", ans + ((n2 - n1) >> 1));
}
}  // namespace Odd
namespace Even {
int s[MAXN], ans;
int C[MAXN], cnt, t[MAXN];
int size[MAXN], a[MAXN];
bool onc[MAXN];
bool findcycle(int u, int f) {
  if (u == B) return true;
  for (int i = st[u]; i; i = nxt[i]) {
    int v = to[i];
    if (v == f) continue;
    if (u == A && v == B) continue;
    C[++cnt] = v;
    if (findcycle(v, u)) return true;
    cnt--;
  }
  return false;
}
int dfs(int u, int f, bool d) {
  int res = d & 1;
  size[u] = 1;
  for (int i = st[u]; i; i = nxt[i]) {
    int v = to[i];
    if (onc[v]) continue;
    if (v == f) continue;
    res += dfs(v, u, d ^ 1);
    size[u] += size[v];
  }
  if (f) ans += abs(size[u] - (res << 1));
  return res;
}
inline void solve() {
  C[cnt = 1] = A;
  findcycle(A, 0);
  for (int i = 1; i <= cnt; i++) onc[C[i]] = true;
  for (int i = 1; i <= cnt; i++)
    s[i] = dfs(C[i], 0, i & 1), t[i] = size[C[i]] - s[i];
  for (int i = 1; i < cnt; i++) {
    s[i] += s[i - 1], t[i] += t[i - 1];
    a[i] = s[i] - t[i];
  }
  s[cnt] += s[cnt - 1], t[cnt] += t[cnt - 1];
  if (s[cnt] != t[cnt]) {
    printf("-1\n");
    return;
  }
  a[cnt] = 0;
  sort(a + 1, a + cnt + 1);
  for (int i = 1; i <= cnt; i++) ans += abs(a[(cnt + 1) >> 1] - a[i]);
  printf("%d\n", ans);
}
}  // namespace Even
void dfs(int u, int fa) {
  vis[u] = true;
  for (int i = st[u]; i; i = nxt[i]) {
    int v = to[i];
    if (v == fa) continue;
    if (vis[v]) {
      A = u, B = v;
      if ((dep[v] & 1) == (dep[u] & 1)) cycle = true;
      continue;
    }
    dep[v] = dep[u] + 1, dfs(v, u);
  }
}
int main() {
  n = read<int>(), m = read<int>();
  if (m == n - 1)
    Tree::solve();
  else {
    for (int i = 1; i <= m; i++) {
      int u = read<int>(), v = read<int>();
      Add(u, v), Add(v, u);
    }
    dfs(dep[1] = 1, 0);
    if (cycle)
      Odd::solve();
    else
      Even::solve();
  }
  return 0;
}
