#include<iostream>
#include<cstring>
using namespace std;
int MAP[1005][1005];
int vis[1005];
int step[1005];
int n,m,s;
struct myqueue
{
    int q[1005];
    int head = 0;
    int tail = 0;
    void Push(int k)
    {
        q[tail++] = k;
    }
    void Pop()
    {
        head++;
    }
    int Front()
    {
        return q[head];
    }
    bool Empty()
    {
        if(head==tail)
        {
            return 1;
        }else
        {
            return 0;
        }
    }

};
void BFS(int s)
{
    myqueue q;
    q.Push(s);
    vis[s] = 1;
    step[s] = 0;
    while(!q.Empty())
    {
        int k = q.Front();
        q.Pop();
        for(int i=1;i<=n;i++)
        {
            if(MAP[k][i]==1&&vis[i]==0)
            {
                q.Push(i);
                step[i] = step[k] + 1;
                vis[i] = 1;
            }
        }
    }
}
int main()
{
    while(cin>>n>>m)
    {
        memset(MAP,0,sizeof(MAP));
        memset(vis,0,sizeof(vis));
        memset(step,0,sizeof(step));
        for(int i=0;i<m;i++)
        {
            int u,v;
            cin>>u>>v;
            MAP[u][v] = 1;
        }
        BFS(n);
        if(vis[1])
        {
            cout<<step[1]<<endl;
        }else
        {
            cout<<"NO"<<endl;
        }
    }
    return 0;
}