#include <bits/stdc++.h>
using namespace std;
namespace mine {
long long qread() {
  long long ans = 0, f = 1;
  char c = getchar();
  while (c < '0' or c > '9') {
    if (c == '-') f = -1;
    c = getchar();
  }
  while ('0' <= c and c <= '9') ans = ans * 10 + c - '0', c = getchar();
  return ans * f;
}
void write(long long num) {
  if (num < 0) putchar('-'), num = -num;
  if (num >= 10) write(num / 10);
  putchar('0' + num % 10);
}
void write1(long long num) {
  write(num);
  putchar(' ');
}
void write2(long long num) {
  write(num);
  putchar('\n');
}
template <typename T>
void chmax(T &x, const T y) {
  x = (x > y ? x : y);
}
template <typename T>
void chmin(T &x, const T y) {
  x = (x < y ? x : y);
}
long long gcd(long long x, long long y) { return y ? gcd(y, x % y) : x; }
const int INF = 0x3f3f3f3f;
const int MOD = 998244353;
inline int mm(const int x) { return x >= MOD ? x - MOD : x; }
template <typename T>
inline void add(T &x, const T y) {
  x = mm(x + y);
}
inline long long qpower(long long x, long long e) {
  long long ans = 1;
  if (e < 0) {
    puts("error");
    exit(666);
  };
  while (e) {
    if (e & 1) ans = ans * x % MOD;
    x = x * x % MOD;
    e >>= 1;
  }
  return ans;
}
long long invm(long long x) { return qpower(x, MOD - 2); }
const int N = 1e3 + 10;
vector<int> to[N];
int dep[N];
int ln, S, T;
int use[N], ct[N], sum;
void calc(int x, int fa) {
  dep[x] = dep[fa] + 1;
  use[x] = (dep[x] & 1 ? 1 : -1);
  sum += use[x];
  for (int t = (0), I = (((int)(to[x]).size()) - 1); t <= I; t++) {
    int y = to[x][t];
    if (y == fa) continue;
    if (dep[y]) {
      if (!ln) ln = dep[x] - dep[y] + 1, S = x, T = y;
    } else
      calc(y, x);
  }
}
int ans;
int pp[N], cnt;
void solve(int x, int fa) {
  for (int t = (0), I = (((int)(to[x]).size()) - 1); t <= I; t++) {
    int y = to[x][t];
    if (y == fa or (x == S and y == T) or (x == T and y == S)) continue;
    solve(y, x);
    use[x] += use[y];
    ct[x] += ct[y];
  }
  if (!ct[x])
    ans += abs(use[x]);
  else
    pp[++cnt] = use[x] * ct[x];
}
void gg() {
  puts("-1");
  exit(0);
}
void main() {
  int n = qread(), m = qread();
  for (int i = (1), I = (m); i <= I; i++) {
    int x = qread(), y = qread();
    to[x].push_back(y);
    to[y].push_back(x);
  }
  calc(1, 0);
  if (m == n - 1) {
    if (sum != 0) gg();
  } else if (ln & 1) {
    if (sum & 1) gg();
    ans += abs(sum / 2);
    use[S] -= sum / 2, use[T] -= sum / 2;
  } else {
    if (sum != 0) gg();
    ct[S]++, ct[T]--;
  }
  solve(1, 0);
  pp[++cnt] = 0;
  sort(pp + 1, pp + cnt + 1);
  for (int i = (1), I = (cnt); i <= I; i++)
    ans += abs(pp[i] - pp[(cnt + 1) / 2]);
  write(ans);
}
};  // namespace mine
signed main() {
  srand(time(0));
  mine::main();
}
