#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
constexpr int Inf = 2000000030;
constexpr ll INF= 2000000000000000001;
constexpr ll MOD = 1000000007;
const double PI = 3.14159265358979323846;
typedef pair<ll,ll> P;
typedef pair<ll,P> PP;

template<typename T> 
vector<T> make_vector(size_t s) {
    return vector<T>(s);
}

template<typename T,typename... Args>
auto make_vector(size_t s,Args... args) {
    return vector(s,make_vector<T>(args...));
}

template<typename T> inline bool chmax(T &a, T b) {
    if (a < b) {
        a = b;
        return 1;
    }
    return 0;
}

template<typename T> inline bool chmin(T &a, T b) {
    if (a > b) {
        a = b;
        return 1;
    }
    return 0;
}

ll mod(ll val, ll M) {
    val = val % M;
    if(val < 0) {
        val += M;
    }
    return val;
}

template<typename T>
T modpow(T N, T P, T M){
    if(P == 0) return 1;
    if(P < 0) return 0;
    if(P % 2 == 0){
        ll t = RS(N, P/2, M);
        if(M == -1) return t * t;
        return t * t % M;
    }
    if(M == -1) return N * modpow(N,P - 1,M);
    return N * modpow(N, P-1, M) % M;
}

struct edge{
    int to;
    ll cost;
};

int N; //頂点数
vector<edge> graph[200010];
ll dist[200010]; //頂点sからの最短距離
int pre[200010];

void Dijkstra(int s) {
    //greater<P>を指定することでfirstが小さい順に取り出せる
    priority_queue<P,vector<P>,greater<P>> que;
    fill(dist,dist + N,INF);
    fill(pre,pre + N,-1);
    dist[s] = 0;
    que.push(P(0,s));

    while(!que.empty()) {
        P p = que.top();
        que.pop();
        int v = p.second;
        if(dist[v] < p.first) {
            continue;
        }
        for(auto x:graph[v]) {
            if(chmin(dist[x.to],dist[v] + x.cost)) {
                pre[x.to] = v;
                que.push(P(dist[x.to],x.to));
            }
        }
    }
}

//最短路を取得
vector<int> get_path(int t) {
    vector<int> path;
    while(t != -1) {
        path.push_back(t);
        t = pre[t];
    }
    reverse(path.begin(),path.end());
    return path;
}

int main() {
    cin >> N;
    int M;
    cin >> M;
    for(int i = 0;i < M;i++) {
        int a,b;
        cin >> a >> b;
        a--;
        b--;
        graph[a].push_back(edge{b,1});
        graph[b].push_back(edge{a,1});
    }
    Dijkstra(0);
    if(dist[N - 1] == INF) cout << -1 << endl;
    else cout << dist[N - 1] << endl;
}