/* -*- coding: utf-8 -*-
 *
 * 1565.cc:  No.1565 Union - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

const int MAX_N = 200000;

/* typedef */

typedef vector<int> vi;
typedef queue<int> qi;

/* global variables */

vi nbrs[MAX_N];
int ds[MAX_N];

/* subroutines */

/* main */

int main() {
  int n, m;
  scanf("%d%d", &n, &m);

  for (int i = 0; i < m; i++) {
    int u, v;
    scanf("%d%d", &u, &v);
    u--, v--;
    nbrs[u].push_back(v);
    nbrs[v].push_back(u);
  }

  memset(ds, -1, sizeof(ds));
  ds[0] = 0;

  qi q;
  q.push(0);

  while (! q.empty()) {
    int u = q.front(); q.pop();
    if (u == n - 1) break;

    for (auto v: nbrs[u])
      if (ds[v] < 0) {
	ds[v] = ds[u] + 1;
	q.push(v);
      }
  }

  printf("%d\n", (ds[n - 1] >= 0) ? ds[n - 1] : -1);
  return 0;
}