I simply did bfs from node 1 and found the maximum distance from this node to all other nodes

const int INF = 1e9;
const int sz = 1e5+1;
int n,m;
vector<int> last(sz);
vector<vector<int> > adj(sz);
vector<int> d(sz,0);
void solve()
{
    cin>>n>>m;

    for(int i=0; i<m; ++i)
    {
        int a,b;
        cin>>a>>b;
        adj[a].push_back(b);
    }

    queue<int> q;
    q.push(1);
    while(!q.empty())
    {
        int s = q.front();
        q.pop();

        for(auto u : adj[s])
        {
            if(d[s]+1>d[u])
            {
                d[u]=1+d[s];
                last[u]=s;
                q.push(u);
            }
        }
    }
    if(d[n]==0)
    {
        cout<<"IMPOSSIBLE"<<endl;
        return;
    }
    vector<int> ans;
    int k=n;
    while(k!=1)
    {
        ans.push_back(k);
        k=last[k];
    }
    ans.push_back(1);
    reverse(ans.begin(), ans.end());
    cout<<1+d[n]<<endl;
    for(int i=0; i<ans.size(); ++i)
        cout<<ans[i]<<" ";
    cout<<endl;
}
int main()
{
    solve();
}