#include "bits/stdc++.h"
using namespace std;
#define all(x) x.begin(), x.end()
#define int long long
typedef priority_queue<array<int, 2>, vector<array<int,2>>, greater<>> prque;
vector<vector<int>> g;
vector<pair<int, int>> tinout;
int tt = 0;
vector<int> dst;
vector<int> dst2, dst3;
vector<int> pap;
void dfst(int v) {
    tinout[v].first = tt++;
    for (auto i : g[v]) {
        if (v == pap[i])
            dfst(i);
    }
    tinout[v].second = tt++;
}
bool ispapa(int papa, int son) {
    return tinout[papa].first <= tinout[son].first and tinout[son].second <= tinout[papa].second;
}
void bfsd(int v) {
    dst.assign(g.size(), -1);
    queue<pair<int, int>> que;
    que.push({v, 0});
    while (!que.empty()) {
        auto [t, prc] = que.front();
        que.pop();
        if (dst[t] != -1) {
            continue;
        }
        dst[t] = prc;
        if (dst[t] == 0)
            pap[t] = t;
        for (auto i : g[t]) {
            if (dst[i] != -1 and dst[i] + 1 == dst[t]) {
                pap[t] = i;
            }
            if (dst[i] == -1)
                que.push({i, dst[t]+1});
        }
    }
}
prque dfscalc(int v) {
    prque a;
    for (auto i : g[v]) {
        if (v == pap[i]) {
            prque eq = dfscalc(i);
            if (a.size() < eq.size()) {
                swap(a, eq);
            }
            while (!eq.empty()) {
                array<int, 2> tq = eq.top();
                eq.pop();
                a.push(tq);
            }
        }
    }
    while (!a.empty()) {
        array<int, 2> tq = a.top();
        if (ispapa(v, tq[1])) {
            a.pop();
        } else {
            break;
        }
    }
    for (auto i : g[v]) {
        if (!ispapa(v, i) and pap[v] != i) {
            a.push({dst[i] + 1 + dst[v], i});
        }
    }
    if (!a.empty()) {
        dst2[v] = a.top()[0] - dst[v];
    }
    return a;
}
void solve() {
    int n, m;
    cin >> n >> m;
    g.assign(n, {});
    tinout.assign(n, {});
    pap.assign(n, {});
    for (int i = 0; i < m; ++i) {
        int a, b;
        cin >> a >> b;
        a--;b--;
        g[a].push_back(b);
        g[b].push_back(a);
    }
    int fin = n-1;
    //cout << "T" << endl;
    bfsd(fin);
    //cout << "T" << endl;
    dfst(fin);
    //cout << "T" << endl;
    dst2.assign(n, -1);
    dfscalc(fin);
    dst3.assign(n, -1);
    //cout << "T" << endl;
    dst2[fin] = 0;
    priority_queue<pair<int, int>, vector<pair<int, int>>, greater<>> quq;
    quq.push({0, fin});
    //cout << "T" << endl;

    while (!quq.empty()) {
        auto [pr, v] = quq.top();
        quq.pop();
        if (dst3[v] != -1)
            continue;
        dst3[v] = pr;
        for (auto i : g[v]) {
            if (dst2[i] == -1)
                continue;
            quq.push({max(dst2[i], pr + 1), i});
        }
    }
    //for (int i = 0; i < n; ++i) {
    //    cout << dst[i] << ' ' << dst2[i] << ' ' << dst3[i] << '\n';
    //}
    cout << dst3[0] << '\n';
}

signed main() {
    ios::sync_with_stdio(0);
    cin.tie(0);cout.tie(0);

    /*
    int x = 4e12;
    int c = 2025;
    int mx = 0;
    for (int i = 0; i < 1000000; ++i) {
        if ((mx = max(mx, solve(x - i * 1345239))) > c) {
            cout << solve(x - i * 1345239) << '\n';
            exit(1);
        }
        cerr << mx << '\n';
    }
    int prc = 0;
     */
    int t = 1;
    //cin >> t;
    while (t--)
        solve();
}