#include <bits/stdc++.h>
#define endl "\n"
using namespace std;
#define int long long int


int n, m;
vector<vector<pair<int,int>>> g;
vector<int> dist;
vector<int> par;

void dij()
{
    priority_queue<pair<int,int>, vector<pair<int,int>>, greater<pair<int,int>>> pq;
    pq.push({0,1});
    dist[1] = 0;
    par[1] = -1;

    while(!pq.empty())
    {
        int u = pq.top().second;
        int d = pq.top().first;
        pq.pop();

        if(dist[u] < d) continue;

        for(auto e: g[u])
        {
            int v = e.first;
            int c = e.second;
            if(dist[v] > dist[u]+c)
            {
                dist[v] = dist[u]+c;
                pq.push({dist[v],v});
                par[v] = u;
            }
        }


    }

}

int32_t main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cin >> n >> m;
    dist.resize(n+1);
    par.resize(n+1);

    for(int i = 1; i <= n; ++i)
    {
        par[i] = -1;
    }

    g.resize(n+1);
    for(int i = 0; i < m; ++i)
    {
        int u, v;
        cin >> u >> v;
        g[u].push_back({v,-1});
    }
    dij();
    if(dist[n] == 0)
    {
        cout << "IMPOSSIBLE";
        return 0;
    }

    cout << 1 + dist[n]*(-1) << endl;
    vector<int> ans;
    int temp = n;
    while(temp != -1)
    {
        ans.push_back(temp);
        temp = par[temp];
    }
    reverse(ans.begin(), ans.end());
    for(auto u: ans)
    {
        cout << u << " ";
    }

}