#include <bits/stdc++.h>

#define int long long
#define For(i,z) for(int i=0;i<z;i++)

using namespace std;

const int N = 5e5+5;
const int INF = 1e12;

vector<int> gr[N];
vector<int> rev[N];

int ans[N][3];

struct node
{
    int flag;
    int dst;
    int v;
    node(){flag = 0; dst = 0; v = 0;}
    bool operator > (const node &ot) const {
        return (dst > ot.dst);
    }
};

int32_t main()
{
    //freopen("input.txt","r", stdin);

    ios_base::sync_with_stdio(false);

    int n, m;
    cin >> n >> m;

    For (i, m)
    {
        int a, b;
        cin >> a >> b;
        a--; b--;
        gr[a].push_back(b);
        rev[b].push_back(a);
        rev[a].push_back(b);
    }

    For (i, N) For (j, 3) ans[i][j] = INF;

    ans[0][0] = 0;

    priority_queue<node, vector<node>, greater<node> > q;

    q.push(node());
    while (!q.empty())
    {
        node cur = q.top();
        q.pop();
        if (ans[cur.v][cur.flag] < cur.dst) continue;

        if (cur.flag == 0)
            for (int &i : rev[cur.v])
                if (ans[i][0] > cur.dst+1)
                {
                    ans[i][0] = cur.dst+1;
                    node nxt; nxt.v = i; nxt.dst = cur.dst+1;
                    q.push(nxt);
                }
        if (cur.flag <= 1)
            for (int &i : gr[cur.v])
                if (ans[i][1] > cur.dst && ans[i][0] > cur.dst)
                {
                    ans[i][1] = cur.dst;
                    node nxt; nxt.v = i; nxt.flag = 1; nxt.dst = cur.dst;
                    q.push(nxt);
                }

        if (cur.flag)
        for (int &i : rev[cur.v])
            if (ans[i][2]>cur.dst+1 && ans[i][1]>cur.dst+1 && ans[i][0] > cur.dst+1)
            {
                ans[i][2] = cur.dst + 1;
                node nxt; nxt.v = i; nxt.flag = 2; nxt.dst = cur.dst + 1;
                q.push(nxt);
            }
    }

    int mn = INF;
    For (i, 3) mn = min(mn, ans[n-1][i]);
    if (mn == INF)
        cout << "-1\n";
    else
        cout << mn;

    return 0;
}