#include <bits/stdc++.h>
using namespace std;
#include <atcoder/all>
using namespace atcoder;
#define rep(i, n) for(int i=0;i<(n);++i)
#define rep1(i, n) for(int i=1;i<=(n);i++)
#define ll long long
using mint = modint998244353;
using P = pair<ll,ll>;
using lb = long double;
using T = tuple<ll, ll, ll>;
#ifdef LOCAL
#  include <debug_print.hpp>
#  define dbg(...) debug_print::multi_print(#__VA_ARGS__, __VA_ARGS__)
#else
#  define dbg(...) (static_cast<void>(0))
#endif

int main()
{
    int n, m;
    cin >> n >> m;
    vector<vector<int>> g(n);
    rep(i,m){
        int a, b;
        cin >> a >> b;
        --a;--b;
        g[a].push_back(b);
    }
    const int INF = 1e9;
    vector<vector<int>> dist(n, vector<int>(1<<n, INF));
    queue<pair<int, int>> q;
    dist[0][0] = 0;
    q.push({0,0});
    while(!q.empty()){
        auto [u, s] = q.front();q.pop();
        for(int v : g[u]){
            int ns = s;
            if(v==n-2) ns|=1;
            if(v==n-1) ns|=2;
            if(dist[v][ns]==INF) {
                dist[v][ns] = dist[u][s] + 1;
                q.emplace(v,ns);
            }
        }
    }
    int ans = dist[0][3];
    if(ans==INF) ans = -1;
    cout << ans << endl;
    return 0;
}