#include <iostream>
#include <cstdio>
#include <cmath>
#include <vector>
#include <algorithm>
#include <queue>
#include <map>
#include <set>
#include <cstring>
#include <utility>
#include <iomanip>

#define ll long long
#define pb push_back

#define INF 2e9 + 10
#define PI acos(-1.0)
#define LL_INF 4e17
#define EPS 1e-9

const int MOD = 1e9 + 7;
const int MAXN = 1e5 + 5;

using namespace std;

void fileIO(string prob) {
    freopen((prob + ".in").c_str(), "r", stdin);
    freopen((prob + ".out").c_str(), "w", stdout);
}

void fastIO() {
    cin.tie(0)->sync_with_stdio(0);
}

// ..... glhf ^-^ .....//

int n, m;
vector<int> adj[100005];
vector<int> radj[100005];
int len[100005], pre[100005];

void dfs(int par, int v, int leng) {
    len[v] = max(len[v], leng);
    pre[v] = par;

    for(int u : adj[v]) {
        if(len[v] + 1 > len[u]) dfs(v, u, leng + 1);
    }
}

int main() {
    cin>>n>>m;
    while(m--) {
        int a, b; cin>>a>>b;
        adj[a].pb(b);
        radj[b].pb(a);
    }

    dfs(-1, 1, 0);
    if(len[n] == 0) {
        cout<<"IMPOSSIBLE";
        return 0;
    }

    int nd = n;

    vector<int> ans;

    for(int i=n ; i != -1 ; i = pre[i]) ans.pb(i);

    cout<<ans.size()<<'\n';
    for(int i=ans.size()-1 ; i>=0 ; i--) {
        cout<<ans[i]<<" ";
    }