/*
Author : Chandan Agrawal
College : Poornima College of Engg. jaipur, Raj
Mail : chandanagrawal23@gmail.com
" when you are not practicing someone else is ,
and the day u meet them u will lose "
*/
#include<bits/stdc++.h>
#include<stdio.h>
using namespace std;
#define fastio ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
#define MAX 300050
#define ll long long
#define ld long double
#define lli long long int
#define pb push_back
#define INF 1000000000000
#define mod 1000000007
// trignometric function always give value in Radians only
#define PI acos(-1) //3.1415926535897932384626433832795028
#define dsin(degree) sin(degree*(PI/180.0))
#define dcos(degree) cos(degree*(PI/180.0))
#define dtan(degree) tan(degree*(PI/180.0))
#define rsin(radian) sin(radian)
#define rcos(radian) cos(radian)
#define rtan(radian) tan(radian)
#define mem0(a) memset(a,0,sizeof(a))
#define mem1(a) memset(a,-1,sizeof(a))
#define memf(a) memset(a,false,sizeof(a))
#define loop(i,n) for (lli i = 0; i < n; i++)
#define FOR(i,a,b) for (lli i = a; i < b; i++)
#define all(v) v.begin(),v.end()
#define rall(v) v.rbegin(),v.rend()
#define makeuniq(v) v.resize(unique(all(v)) - v.begin()); //only uniq element in vector after this
#define sz(x) int(x.size())
#define F first
#define S second
#define mii map<lli,lli>
#define pii pair<lli,lli>
#define vi vector<lli>
#define vvi vector<vi>
#define vpi vector<pii>
#define vbool vector<bool>
#define seti set<lli>
#define gcd(a,b) __gcd((a),(b))
#define lcm(a,b) (a/gcd(a,b))*b
#define abs(x) ((x < 0)?-(x):x)
#define endl '\n'
template <typename Head>
void print(Head&& head)
{
cout<<head<<endl;
}
template <typename Head, typename... Tail>
void print(Head&& head, Tail... tail)
{
cout<<head<<" ";
print(tail...);
}
#define scanarr(a,n) for(lli i=0;i<n;i++) cin>>a[i];
#define scanvec(a,n) for(lli i=0;i<n;i++){ lli x ; cin>>x; a.pb(x);}
#define printarr(a,n) for(lli i=0;i<n;i++) cout<<a[i]<<" "; cout<<endl;
#define printvec(vec) for(auto xt : vec) cout<<xt<<" "; cout<<"\n";
#define FD(N) fixed<<setprecision(N)
#define deb(x) cout<<#x<<" "<<x<<endl;
/*
1D vector - vi dp(n,value);
2D vector - vvi dp(n,vi(n,value));
*/
// chandan1,2
void chandan1(){int y=1;return;}
void chandan2(){
loop(i,10){
lli x=1;
}
return(chandan1());
}
//-------------------------------------------------------------GRAPH--------------------------------------------------------------------
int vis[MAX];
vi adj[MAX];
lli parent[MAX];
lli depth[MAX];
lli dp[MAX];//subtree size
lli color[MAX];
lli cnt=0;
lli indegree[MAX];
void clear(lli n)
{
loop(i,n+1)
{
vis[i]=0;
adj[i].clear();
parent[i]=0;
depth[i]=0;
dp[i]=0;
}
}
void dfs(lli src)
{
vis[src]=1;
dp[src]=1;
cnt+=1;
for(auto xt : adj[src])
{
if(!vis[xt])
{
dfs(xt);
dp[src]+=dp[xt];
}
}
}
bool bfs(lli src)
{
vis[src]=1;
queue<lli>q;
q.push(src);
depth[src]=0;
parent[src]=src;
color[src]=1;
while(!q.empty())
{
lli u = q.front();q.pop();
for(auto xt : adj[u])
{
if(!vis[xt])
{
vis[xt]=1;
q.push(xt);
parent[xt]=u;
depth[xt] = depth[u]+1;
color[xt] = 1-color[u];
}
if(color[xt] == color[u])
return false;
}
}
return true;
}
//https://g...content-available-to-author-only...b.com/chandanagrawal23/Data-Structures/blob/master/Topological%20Sort_BFS
// we can use either queue or priority que a/c to question
// if sz(ans) != v then it means it contains cycle
vi toposort(lli n){
//priority_queue<lli , vi , greater<int> > pq;
queue<lli> pq;
vi ans;
for(lli i=1;i<=n;i++)
if(indegree[i] == 0)
pq.push(i);
while(!pq.empty())
{
lli cur = pq.front();
ans.pb(cur);
pq.pop();
for(auto node : adj[cur])
{
indegree[node]--;
if(indegree[node]==0)
pq.push(node);
}
}
return ans;
}
//-------------------------------------------------------------GRAPH--------------------------------------------------------------------
int main(){
fastio
lli t=1;
//cin>>t;
chandan2();
while(t--) {
lli n,m,k;
cin>>n>>m;
loop(i,m)
{
lli x,y;
cin>>x>>y;
adj[x].pb(y);
indegree[y]++;
}
vi topo_order = toposort(n);
if(sz(topo_order)!=n) //if cycle is present then answer will be infinite so that print -1
print("IMPOSSIBLE");
else
{
lli dist[n+1]={0};
lli parent[n+1];
FOR(i,1,n+1)
parent[i] = i;
for(auto x : topo_order)
{
for(auto y : adj[x])
{
if(dist[y]<(dist[x]+1))
{
dist[y] = dist[x]+1;
parent[y] = x;
}
}
}
vi ans;
ans.pb(n);
while(parent[n]!=n)
{
n = parent[n];
ans.pb(n);
}
reverse(all(ans));
if(ans[0]==1)
{
print(sz(ans));
printvec(ans);
}
else
print("IMPOSSIBLE");
}
}
return 0;
}