csuoj 1256

#include<iostream>
#include<cstring>
#include<algorithm>
#include<queue>
using namespace std;
#define MAX 20010
#define inf 0x3f3f3f3f
int n,m;
int first[MAX],d[MAX],inq[MAX],w[MAX],v[MAX],next[MAX],e;

void init()
{
    e =0 ;
    memset(first,-1,sizeof(first));
}

void add_edge(int a,int b,int c)
{
    v[e] = b;
    next[e] = first[a];
    w[e] = c;
    first[a]= e++;
}
void spfa(int src)
{
    queue<int>q;
    memset(d,0x3f,sizeof(d));
    d[src]= 0,inq[src] = 1;
    q.push(src);
    while(!q.empty())
    {
        int u = q.front();
        q.pop();
        inq[u] =0;
        for(int i =first[u];i!= -1;i = next[i])
        {
            if(d[v[i]]>d[u]+w[i])
            {
                d[v[i]] = d[u]+ w[i];
                if(!inq[v[i]])q.push(v[i]),inq[v[i]] =1;
            }
        }
    }
}

int main()
{
    while(cin>> n>> m)
    {
        init();
        for(int i=0;i< m;i++)
        {
            int a,b;
            cin >> a>>b;
            add_edge(a,b,0);
            add_edge(b,a,1);
        }
        spfa(1);
        if(d[n]!= inf)cout<<d[n]<< endl;
        else cout<< "-1"<< endl;
    }
    return 0;
}