#include<iostream>

#include<cstdio>

#include<cmath>

#include<algorithm>

#include<cstring>

#include<string>

#include<cstdlib>

#include<vector>

#include<stack>

#include<map>

using namespace std;

typedef long long ll;

const int INF=10e8;

const int MAXN=410;

int k,minn;

int lc[MAXN],c1[MAXN][MAXN],c2[MAXN][MAXN];

bool vis[MAXN];

int dij(int n,int c[][MAXN])

{

    memset(vis,0,sizeof(vis));

    for(int i=1;i<=n;i++)

        lc[i]=c[1][i];

    vis[1]=1;

    for(int j=1;j<=n;j++)

    {

        k=-1,minn=INF;

        for(int i=1;i<=n;i++)

        {

            if(!vis[i]&&minn>lc[i])

            {

                minn=lc[i];

                k=i;

            }

        }

        if(k==-1) break;

        vis[k]=1;

        for(int i=1;i<=n;i++)

            if(!vis[i]&&lc[i]>lc[k]+c[k][i])

                lc[i]=lc[k]+c[k][i];

    }

    if(lc[n]==INF)

        lc[n]=-1;

    return lc[n];

}

int main()

{

    int n,m;

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

    int flag=0;

    for(int i=1;i<=n;i++)

    {

        for(int j=1;j<=n;j++)

        {

            if(i==j)

            {

                c1[i][j]=c1[j][i]=0;

            }

            else

            {

                c1[i][j]=c1[j][i]=INF;

                c2[i][j]=c2[j][i]=INF;

            }

        }

    }

    for(int i=1;i<=m;i++)

    {

        int u,v;

        scanf("%d%d",&u,&v);

        c1[u][v]=c1[v][u]=1;

        if((u==1&&v==n)||(u==n&&v==1))

            flag=1;

    }

    for(int i=1;i<=n;i++)

    {

        for(int j=1;j<=n;j++)

        {

            if(c1[i][j]==0||c1[i][j]==INF)

                c2[i][j]=c2[j][i]=1;

        }

    }

    if(m==n*(n-1)/2)

        printf("-1\n");

    else

    {

        if(flag==1)

            printf("%d\n",dij(n,c2));

        else if(flag==0)

            printf("%d\n",dij(n,c1));

    }

    return 0;

}