Shared code

//PPAP_1264589
#include <bits/stdc++.h>
#define Task "A"
#define up(i,a,b) for (int i = a; i <= b; i++)
#define ep emplace_back
using namespace std;

const int maxn = 1e5 + 10;
int dp[maxn];
int existed[maxn];
int tr[maxn];
int deg[maxn];
bool inchain[maxn];
vector<int> a[maxn];
int n,m;

void in(){
    cin >> n >> m;
    up(i,1,m){
        int u,v;
        cin >> u >> v;
        if (existed[u] == v) continue;
        existed[u] = v;
        a[u].ep(v);
        ++deg[v];
    }
}

queue<int> Q;
void topo_sort(){
    up(i,1,n) if (deg[i] == 0) Q.push(i);
    while (!Q.empty()){
        int u = Q.front();
        Q.pop();
        for (int v : a[u]){
            if (dp[v] < dp[u] + 1 && inchain[u]){
                dp[v] = dp[u] + 1;
                tr[v] = u;
                inchain[v] = 1;
            }
            if (--deg[v] == 0) Q.push(v);
        }
    }
}

void Fpath(){
    int x = n;
    vector<int> path;
    while (x != 1){
        path.ep(x);
        x = tr[x];
    }
    path.ep(1);
    reverse(path.begin(), path.end());
    for (auto x : path) cout << x << " ";
}

signed main(){
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    if (fopen(Task".inp", "r")){
        freopen (Task".inp", "r", stdin);
        freopen (Task".out", "w", stdout);
    }

    in();
    inchain[1] = 1;
    dp[1] = 1;
    topo_sort();
    if (!inchain[n]){
        cout << "IMPOSSIBLE";
        return 0;
    }
    cout << dp[n] << "\n";
    Fpath();
}

// Algorithmic logic