Shared code

#include <bits/stdc++.h>

using namespace std;

//#pragma GCC optimize("Ofast","unroll-loops","omit-frame-poller","inline","03")

#define all(x) (x).begin(), (x).end()

typedef long double ld;
typedef long long ll;
typedef pair <int, int> ii;
typedef __int128 LL;

vector <int> dp, sol;
vector <vector <int>> ady;

int solve(int x) {
    int &aux = dp[x];
    if(aux != -1)return aux;
    aux = 0;
    for(int to : ady[x])
        aux = max(aux, solve(to));
    if(aux)aux++;
    return aux;
}

int n, m;

void dfs(int x) {
    cout << x;
    if(x == n) {
        cout << '\n';
        return;
    }
    cout << ' ';
    for(int to : ady[x])
        if(dp[x] == dp[to] + 1) {
            dfs(to);
            return;
        }
}

int x, y;

int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0);

    cin >> n >> m;

    dp.resize(n + 1, -1);
    ady.resize(n + 1, vector <int> ());

    while(m--) {
        cin >> x >> y;
        ady[x].push_back(y);
    }

    dp[n] = 1;
    int sol = solve(1);

    if(!sol) {
        cout << "IMPOSSIBLE\n";
        return 0;
    }

    cout << sol << '\n';
    dfs(1);

    return 0;
}