#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef long double ld;
typedef pair<int,int> pii;

#define fr first
#define sc second

#define vc vector
#define pb push_back

#define all(x) x.begin() , x.end()

#define Heh ios::sync_with_stdio(false) , cin.tie(0)

/*
const int MAXN = 1e3 + 13;
const ld err = 1e-12;

int par[MAXN] , rnk[MAXN];

int get(int v){return par[v] = (par[v] == v ? v : get(par[v]));}
void uni(int a , int b){
  a = get(a) , b = get(b);
  if(a == b) return;
  if(rnk[a] < rnk[b]) swap(a , b);
  par[b] = a;
  rnk[a] += rnk[b];
}

ld dis(ld a , ld b , ld c , ld d){
  return sqrt((a - c) * (a - c) + (b - d) * (b - d));
}
*/

ll pw(ll a , ll b , ll mod){
  ll ret = 1;
  while(b){
    if(b&1) ret = (ret * a) % mod;
    a = (a * a) % mod;
    b /= 2;
  }
  return ret;
}

/*
const int MAXN = 3e5 + 35 , p = 313 , mod = 1e9 + 7;

string s;
int n , hsh[MAXN] , pp[MAXN] , inv[MAXN];

int hshsub(int l , int r){
  int ret = hsh[r];
  if(l){
    ret -= hsh[l-1];
    if(ret < 0) ret += mod;
  }
  ret = (1ll * ret * inv[l]) % mod;
  return ret;
}
*/

int main(){
  Heh;
  int n , m;
  cin >> n >> m;
  vc<pii> adj[n];
  for(int i = 0 ; i < m ; i++){
    int a , b , c;
    cin >> a >> b;
    c = 1;
    a-- , b--;
    adj[a].pb({b , c});
    adj[b].pb({a , c});
  }
  int dis[n];
  fill(dis , dis + n , 1e9);
  priority_queue<pii , vc<pii> , greater<pii>> pq;
  dis[0] = 0;
  pq.push({dis[0] , 0});
  while(pq.size()){
    auto [dv , v] = pq.top();
    pq.pop();
    if(dis[v] != dv) continue;
    for(auto [u , w] : adj[v]){
      if(dis[u] > dis[v] + w){
	dis[u] = dis[v] + w;
	pq.push({dis[u] , u});
      }
    }
  }
  int ans = 0;
  for(int i = 0 ; i < n ; i++) ans = max(ans , dis[i]);
  if(ans == 1e9) cout << -1 << endl;
  else{
    int oans = 0;
    while(ans > 1) ans = (ans + 1) / 2 , oans++;
    cout << oans + 1 << endl;
  }
}