#include <iostream>

 #include <stdio.h>

 #include <string.h>

 #define INF 0x3f3f3f3f

 using namespace std;

 int rail[][];

 int road[][];

 int n, m;

 int dijkstra(int town[][])

 {

     int dist[];

     bool use[];

     memset(use, , sizeof(use));

     fill(dist, dist+n+, INF);

     dist[] = ;

     while (true)

     {

         int v = -;

         for (int i = ; i <= n; i++)

         {

             if (!use[i] && (v == - || dist[i] < dist[v])) v = i;

         }

         if (v == -) break;

         use[v] = true;

         for (int i = ; i <= n; i++)

         {

             dist[i] = min(dist[i], dist[v] + town[v][i]);

         }

     }

     return dist[n];

 }

 int main()

 {

     freopen("in.txt", "r", stdin);

     scanf("%d%d", &n, &m);

     for (int i = ; i <= n; i++)

     {

         for (int j = ; j <= n; j++)

         {

             rail[i][j] = INF;

             road[i][j] = ;

         }

     }

     for (int i = ; i < m; i++)

     {

         int x, y;

         scanf("%d%d", &x, &y);

         rail[x][y] = ;

         rail[y][x] = ;

         road[x][y] = INF;

         road[y][x] = INF;

     }

     int ans = ;

     if (rail[][n] == INF || rail[n][] == INF) ans = dijkstra(rail);

     else ans = dijkstra(road);

     if (ans == INF) printf("-1\n");

     else printf("%d\n",ans);

 }