#include <bits/stdc++.h>

#define For(i, a, b) for (int i = (int)(a); i < (int)(b); ++i)
#define rFor(i, a, b) for (int i = (int)(a)-1; i >= (int)(b); --i)
#define rep(i, n) For((i), 0, (n))
#define rrep(i, n) rFor((i), (n), 0)
#define fi first
#define se second

using namespace std;

typedef long long lint;
typedef unsigned long long ulint;
typedef pair<int, int> pii;
typedef pair<lint, lint> pll;

template <class T>
bool chmax(T &a, const T &b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}
template <class T>
bool chmin(T &a, const T &b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}

template <class T>
T div_floor(T a, T b) {
    if (b < 0) a *= -1, b *= -1;
    return a >= 0 ? a / b : (a + 1) / b - 1;
}
template <class T>
T div_ceil(T a, T b) {
    if (b < 0) a *= -1, b *= -1;
    return a > 0 ? (a - 1) / b + 1 : a / b;
}

template <typename T>
struct coord_comp {
    vector<T> v;
    bool sorted = false;

    coord_comp() {}

    int size() { return v.size(); }

    void add(T x) { v.push_back(x); }

    void build() {
        sort(v.begin(), v.end());
        v.erase(unique(v.begin(), v.end()), v.end());
        sorted = true;
    }

    int get_idx(T x) {
        assert(sorted);
        return lower_bound(v.begin(), v.end(), x) - v.begin();
    }

    T &operator[](int i) { return v[i]; }
};

constexpr lint mod = 1000000007;
constexpr lint INF = mod * mod;
constexpr int MAX = 100010;

template <typename T>
struct edge {
    int from, to, id;
    T cost;
    edge(int f, int t, T c, int i = 0) : from(f), to(t), cost(c), id(i) {}
};

template <typename T>
struct Graph {
    vector<vector<edge<T>>> G;
    int n;

    Graph(int n_) : n(n_) { G.resize(n); }

    void add_edge(int f, int t, T c, int i = 0) {
        G[f].emplace_back(f, t, c, i);
    }
};

template <typename T>
pair<vector<T>, vector<T>> bfs(Graph<T> &gr, int s) {
    vector<T> d(gr.n, numeric_limits<T>::max()), p(gr.n, -1);
    queue<int> que;
    d[s] = 0;
    que.push(s);
    while (!que.empty()) {
        int v = que.front();
        que.pop();
        for (auto &e : gr.G[v]) {
            if (chmin(d[e.to], d[v] + e.cost)) {
                p[e.to] = v;
                que.push(e.to);
            }
        }
    }
    return {d, p};
}

int main() {
    int n, m;
    scanf("%d%d", &n, &m);
    Graph<int> gr(n);
    rep(i, m) {
        int a, b;
        scanf("%d%d", &a, &b);
        --a;
        --b;
        gr.add_edge(a, b, 1);
        gr.add_edge(b, a, 1);
    }
    auto d = bfs(gr, 0).fi;
    printf("%d\n", d[n - 1] < mod ? d[n - 1] : -1);
}