#include <bits/stdc++.h>
#include <iostream>
using namespace std;

#define endl '\n'
#define int long long
#define ull unsigned long long
typedef long long ll;
#define lowbit(x) ((x) & -(x))

const ll INF = 0x3f3f3f3f;
const ll mod = 1e9 + 7;
const int N = 1e6 + 5, M = 5e5 + 10;
typedef pair<int, int> PII;
double T = 1 >> 30;
// double PI = acos(-1);

int gcd(int a, int b)
{
    return b ? gcd(b, a % b) : a;
}

vector<int> e[N];
int dis[N];

void solve()
{
    int i, j;

    int n, m;
    cin >> n >> m;

    for (i = 1; i <= m; i++)
    {
        int u, v;
        cin >> u >> v;
        e[u].push_back(v);
        e[v].push_back(u);
    }

    for (i = 1; i <= n; i++)
    {
        dis[i] = 1e9;
    }

    queue<int> q;
    dis[1] = 0;
    q.push(1);
    while (!q.empty())
    {
        int u = q.front();
        q.pop();
        for (int v : e[u])
        {
            if (dis[v] > dis[u] + 1)
            {
                dis[v] = dis[u] + 1;
                q.push(v);
            }
        }
    }

    int mx = 0;
    for (i = 1; i <= n; i++)
    {
        mx = max(mx, dis[i]);
    }

    if (mx == 1e9)
    {
        cout << -1;
        return;
    }

    int ans = ceil(log2(mx)) + 1;
    cout << ans;
}

signed main()
{
     ios_base::sync_with_stdio(false);
     cin.tie(nullptr);
     cout.tie(nullptr);
    int t = 1;

    // cin >> t;

    while (t--)
    {
        //  Case++;
        solve();
    }

    return 0;
}