#include <bits/stdc++.h>
using namespace std;
const int N = 100005;
const int inf = 1e9 + 7;
int n, m, f[N][2];
vector<int> g[N];
void dfs(int v, int pr = -1) {
  f[v][0] = 0;
  f[v][1] = inf;
  for (auto& to : g[v]) {
    if (to == pr) continue;
    dfs(to, v);
    int b[2] = {f[v][0], f[v][1]};
    f[v][0] = f[v][1] = inf;
    for (int i = 0; i < 2; ++i) {
      f[v][i ^ 1] = min(f[v][i ^ 1], b[i] + f[to][0] + 1);
      f[v][i] = min(f[v][i], b[i] + f[to][1]);
    }
  }
}
int main() {
  scanf("%d %d", &n, &m);
  if (n == m) {
    return 0;
  }
  for (int i = 0; i < m; ++i) {
    int x, y;
    scanf("%d %d", &x, &y);
    --x;
    --y;
    g[x].push_back(y);
    g[y].push_back(x);
  }
  dfs(0);
  cout << (f[0][1] == inf ? -1 : f[0][1]) << endl;
  return 0;
}
