#include <bits/stdc++.h>
using namespace std;
#define rep(i, s, n) for (int i = (s); i < (int)(n); i++)
#include <atcoder/all>
using namespace atcoder;
using mint1 = modint1000000007;
using mint2 = modint998244353;
using ll = long long;
using ull = unsigned long long;
using ld = long double;
ld pi=3.141592653589793;
const int inf=2e9;
const ll linf=2e18;
const ld eps=1e-14;
#define Yes cout << "Yes" << endl
#define No cout << "No" << endl

int main() {
    ll n,m;
    cin >> n >> m;
    vector<vector<ll>> G(n);
    for(ll i=0; i<m; i++) {
        ll u,v;
        cin >> u >> v;
        u--,v--;
        G[u].push_back(v);
    }

    vector<ll> dist(n,-1);
    dist[0]=0;
    queue<ll> que;
    que.push(0);
    while(!que.empty()) {
        ll v=que.front();
        que.pop();
        for(auto nv:G[v]) {
            if(dist[nv]!=-1) {
                continue;
            }else {
                dist[nv]=dist[v]+1;
                que.push(nv);
            }
        }
    }

    vector<ll> dist2(n,-1);
    dist2[n-1]=0;
    que.push(n-1);
    while(!que.empty()) {
        ll v=que.front();
        que.pop();
        for(auto nv:G[v]) {
            if(dist2[nv]!=-1) {
                continue;
            }else {
                dist2[nv]=dist2[v]+1;
                que.push(nv);
            }
        }
    }

    vector<ll> dist3(n,-1);
    dist3[n-2]=0;
    que.push(n-2);
    while(!que.empty()) {
        ll v=que.front();
        que.pop();
        for(auto nv:G[v]) {
            if(dist3[nv]!=-1) {
                continue;
            }else {
                dist3[nv]=dist3[v]+1;
                que.push(nv);
            }
        }
    }

    ll ans=linf;
    if(dist[n-2]!=-1 && dist3[n-1]!=-1 && dist2[0]!=-1) {
        ans=min(dist[n-2]+dist3[n-1]+dist2[0],ans);
    }
    if(dist[n-1]!=-1 && dist2[n-2]!=-1 && dist3[0]!=-1) {
        ans=min(dist[n-1]+dist2[n-2]+dist3[0],ans);
    }
    if(dist[n-1]!=-1 && dist2[n-2]!=-1 && dist3[n-1]!=-1 && dist2[0]!=-1) {
        ans=min(dist[n-1]+dist2[n-2]+dist3[n-1]+dist2[0],ans);
    }
    if(dist[n-2]!=-1 && dist3[n-1]!=-1 && dist2[n-2]!=-1 && dist3[0]!=-1) {
        ans=min(dist[n-2]+dist3[n-1]+dist2[n-2]+dist3[0],ans);
    }

    if(ans==linf) {
        cout << -1 << endl;
    }else {
        cout << ans << endl;
    }
}