import std;

void main () {
    int N, M; readln.read(N, M);
    int[][] graph = new int[][](N, 0);
    foreach (i; 0..M) {
        int u, v; readln.read(u, v);
        u--, v--;
        graph[u] ~= v;
    }

    solve(N, M, graph);
}

void solve (int N, int M, int[][] graph) {
    /* 最適ルートは 1 -> N-1 -> N -> 1 or 1 -> N -> N-1 -> 1のどちらか。これを全部BFSで調べる */
    int[] visited = new int[](N);
    DList!int Q;
    void BFS (int begin) {
        visited[] = int.max;
        visited[begin] = 0;
        Q.insertBack(begin);

        while (!Q.empty) {
            auto head = Q.front; Q.removeFront;
            foreach (to; graph[head]) {
                if (visited[to] == int.max) {
                    visited[to] = visited[head] + 1;
                    Q.insertBack(to);
                }
            }
        }

    }

    int ans = int.max;
    { // 1 -> N-1 -> N -> 1
        long candi = 0;
        BFS(0);
        candi += visited[N-2];
        BFS(N-2);
        candi += visited[N-1];
        BFS(N-1);
        candi += visited[0];

        if (candi < int.max) ans = min(ans, cast(int) candi);
    }
    { // 1 -> N -> N-1 -> 1
        long candi = 0;
        BFS(0);
        candi += visited[N-1];
        BFS(N-1);
        candi += visited[N-2];
        BFS(N-2);
        candi += visited[0];

        if (candi < int.max) ans = min(ans, cast(int) candi);
    }

    if (ans == int.max) {
        writeln(-1);
    }
    else {
        writeln(ans);
    }
}

void read (T...) (string S, ref T args) {
    auto buf = S.split;
    foreach (i, ref arg; args) {
        arg = buf[i].to!(typeof(arg));
    }
}