#include <bits/stdc++.h>

using namespace std;

const int inf = int(1e9);
const int N = int(2e5) + 10;
const int M = 2 * 30 * N;

int pref[N], suff[N], val[M], ch[M][2], tsz;

void Modify(int& x, int px, int l, int r, int p, int v) {
  x = ++tsz;
  ch[x][0] = ch[px][0];
  ch[x][1] = ch[px][1];
  if (px == 0) {
    val[x] = v;
  } else {
    val[x] = min(val[px], v);
  }
  if (l == r) {
    return;
  }
  int mid = l + r >> 1;
  if (p <= mid) {
    Modify(ch[x][0], ch[px][0], l, mid, p, v);
  } else {
    Modify(ch[x][1], ch[px][1], mid + 1, r, p, v);
  }
}

int Query(int x, int l, int r, int ll, int rr) {
  if (ll <= l && r <= rr) {
    return (x == 0 ? inf : val[x]);
  }
  int mid = l + r >> 1;
  if (rr <= mid) {
    return Query(ch[x][0], l, mid, ll, rr);
  } else if (ll > mid) {
    return Query(ch[x][1], mid + 1, r, ll, rr);
  } else {
    return min(Query(ch[x][0], l, mid, ll, rr), Query(ch[x][1], mid + 1, r, ll, rr));
  }
}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n, m;
  cin >> n >> m;
  vector<vector<int>> g(n);
  for (int i = 0; i < m; i++) {
    int u, v;
    cin >> u >> v;
    --u; --v;
    g[u].push_back(v);
    g[v].push_back(u);
  }
  vector<int> que(1, n - 1);
  vector<int> dist(n, -1);
  dist.back() = 0;
  vector<vector<int>> nei(n);
  vector<vector<int>> back(n);
  vector<int> pr(n, -1);
  for (int b = 0; b < int(que.size()); b++) {
    int i = que[b];
    for (int j : g[i]) {
      if (dist[j] == -1) {
        dist[j] = dist[i] + 1;
        que.push_back(j);
        nei[i].push_back(j);
        nei[j].push_back(i);
        pr[j] = i;
      } else if (j != pr[i]) {
        back[i].push_back(j);
      }
    }
  }
  if (pr[0] == -1) {
    cout << -1 << '\n';
    return 0;
  }
  vector<int> d0(n, -1);
  {
    d0[0] = 0;
    vector<int> que(1, 0);
    for (int b = 0; b < int(que.size()); b++) {
      int i = que[b];
      for (int j : g[i]) {
        if (d0[j] == -1) {
          d0[j] = d0[i] + 1;
          que.push_back(j);
        }
      }
    }
  }
  vector<int> who(n);
  vector<int> dfn(n);
  vector<int> dfo(n);
  int T = -1;
  auto Dfs = [&](auto&& self, int v, int pv) -> void {
    dfn[v] = ++T;
    who[T] = v;
    for (int u : nei[v]) {
      if (u == pv) {
        continue;
      }
      self(self, u, v);
    }
    dfo[v] = T;
  };
  Dfs(Dfs, n - 1, -1);
  for (int t = 0; t < n; t++) {
    if (t > 0) {
      pref[t] = pref[t - 1];
    }
    int i = who[t];
    for (int to : back[i]) {
      Modify(pref[t], pref[t], 0, n - 1, dfn[to], dist[to] + dist[i]);
    }
  }
  for (int t = n - 1; t >= 0; t--) {
    if (t + 1 < n) {
      suff[t] = suff[t + 1];
    }
    int i = who[t];
    for (int to : back[i]) {
      Modify(suff[t], suff[t], 0, n - 1, dfn[to], dist[to] + dist[i]);
    }
  }
  auto Best = [&](int v, int l, int r) {
    int ret = inf;
    if (l > 0) {
      ret = min(ret, Query(pref[l - 1], 0, n - 1, l, r) + 1 - dist[v]);
    }
    if (r + 1 < n) {
      ret = min(ret, Query(suff[r + 1], 0, n - 1, l, r) + 1 - dist[v]);
    }
    return ret;
  };
  vector<int> val(n);
  for (int i = 0; i < n; i++) {
    val[i] = Best(i, dfn[i], dfo[i]);
  }
  val[n - 1] = 0;
  function<bool(int)> Check = [&](int t) {
    if (val[0] > t) {
      return false;
    }
    vector<int> que(1, 0);
    vector<int> dist(n, -1);
    dist[0] = 0;
    for (int b = 0; b < int(que.size()); b++) {
      int i = que[b];
      for (int j : g[i]) {
        if (dist[j] == -1 && (val[j] + dist[i] + 1) <= t) {
          dist[j] = dist[i] + 1;
          que.push_back(j);
        }
      }
    }
    if (dist[n - 1] != -1 && dist[n - 1] <= t) {
      return true;
    } else {
      return false;
    }
  };
  int low = 0, high = inf, res = inf;
  while (low <= high) {
    int mid = low + high >> 1;
    if (Check(mid)) {
      res = mid;
      high = mid - 1;
    } else {
      low = mid + 1;
    }
  }
  cout << (res == inf ? -1 : res) << '\n';
  return 0;
}