/***************************************************
 * Problem Name : F - Pure.cpp
 * Problem Link : https://atcoder.jp/contests/abc142/tasks/abc142_f
 * OJ           : atCoder
 * Verdict      : Trying
 * Date         : 2020-01-04
 * Problem Type : ABC - F
 * Author Name  : Saikat Sharma
 * University   : CSE, MBSTU
 ***************************************************/
#include <iostream>
#include <cstdio>
#include <cmath>
#include <algorithm>
#include <climits>
#include <cstring>
#include <string>
#include <sstream>
#include <vector>
#include <queue>
#include <list>
#include <unordered_map>
#include <unordered_set>
#include <cstdlib>
#include <deque>
#include <stack>
#include <bitset>
#include <cassert>
#include <map>
#include <set>
#include <cassert>
#include <iomanip>
#include <random>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

using namespace std;
using namespace __gnu_pbds;
typedef long long ll;
typedef unsigned long long ull;

#define __FastIO ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0)
#define __FileRead freopen ("input.txt", "r", stdin)
#define __FileWrite freopen ("output.txt", "w", stdout)
#define SET(a,v) memset(a,v,sizeof(a))
#define SZ(v) (int)v.size()
#define pii pair<int,int>
#define pil pair <int, ll>
#define pli pair <ll, int>
#define pll pair <ll, ll>
#define debug cout <<"######\n"
#define debug1(x) cout <<"### " << x << " ###\n"
#define debug2(x,y) cout <<"# " << x <<" : "<< y <<" #\n"
#define nl cout << "\n";
#define sp cout << " ";
#define sl(n) scanf("%lld", &n)
#define sf(n) scanf("%lf", &n)
#define si(n) scanf("%d", &n)
#define ss(n) scanf("%s", n)
#define pf(n) scanf("%d", n)
#define pfl(n) scanf("%lld", n)
#define all(v) v.begin(), v.end()
#define rall(v) v.begin(), v.end()
#define srt(v) sort(v.begin(), v.end())
#define r_srt(v) sort(v.rbegin(), v.rend())
#define rev(v) reverse(v.begin(), v.end())
#define Sqr(x) ((x)*(x))
#define Mod(x, m) ((((x) % (m)) + (m)) % (m))
#define max3(a, b, c) max(a, max(b, c))
#define min3(a, b, c) min(a, min(b, c))
#define pb push_back
#define mk make_pair
#define MAX 1005
#define INF 1000000009
#define MOD 1000000007

template<class T>
using min_heap = priority_queue<T, std::vector<T>, std::greater<T>>;
template<typename T>
using ordered_set  = tree<T, null_type, less<T>, rb_tree_tag,
      tree_order_statistics_node_update>;

template <typename T> string toString ( T Number ) {
    stringstream ss;
    ss << Number;
    return ss.str();
}
template<typename T> int toInt (T str) {
    stringstream ss;
    ss << str;
    int num;
    ss >> num;
    return num;
}
ll lcm (ll a, ll b) {
    return a * b / __gcd (a, b);
}
/************************************ Code Start Here ******************************************************/
vector<int>adj[MAX];
int par[MAX];
int vis[MAX], dis[MAX];
int c;
void dfs (int u) {
    vis[u] = 1;

    for (int v : adj[u]) {
        if (vis[v] == 0) {
            dfs (v);

        } else if (vis[v] == 1) {
            c = v;
            return;
        }
    }

    vis[u] = 2;
}

void bfs (int s) {
    SET (vis, 0);
    SET (dis, 0);
    SET (par, -1);
    queue<int>q;
    q.push (s);
    vis[s] = 1;

    dis[s] = 0;
    while (!q.empty() ) {
        //~ debug;
        int u = q.front();
        q.pop();
        for (int v : adj[u]) {
            if (v == s) {
				dis[v] = dis[u]+1;
                par[v] = u;
                return;

            } else if(vis[v] == 0){
				dis[v] = dis[u] + 1;
                vis[v] = 1;
                par[v] = u;
                q.push (v);
            }
        }
    }
}

int main () {
    //~ __FastIO;
    //~ cout << setprecision (10);
    //~ cout << fixed;
    int n, m;
    cin >> n >> m;
    map<pii, int>mp;

    for (int i = 0; i < m; i++) {
        int x, y;
        cin >> x >> y;
        mp[ {x, y}] = i + 1;
        adj[x].pb (y);
    }

    c = -1;
    SET (vis, 0);
    

    for (int i = 1; i <= n; i++) {
        if (vis[i] == 0) dfs (i);
    }

    if (c == -1) {
        cout << -1 << "\n";

    } else {
        //~ debug;
        //~ cout << c << "  -\n";
        bfs (c);
        //~ cout << c << " --\n";
        //~ debug;
        vector<int>path;
        int u = c;
        path.pb (u);

        while (1) {
            //~ debug;
            u = par[u];

            if (u == c) break;

            path.pb (u);
        }
        //~ cout << (int) path.size() << "\n";
        int mn = INF;
        u = -1;
        for (int i = 0; i < (int) path.size(); i++) {
			int x = path[i];
			bfs(x);
			//~ cout << x << " : " << dis[x] << "\n";
			if(dis[x]<mn){
				mn = dis[x];
				u = x;
			}
            //~ cout << path[i] << "\n";
        }
        bfs(u);
        //~ debug;
        vector<int>ans;
        c = u;
        ans.pb (u);

        while (1) {
			//~ debug;
            //~ debug;
            u = par[u];

            if (u == c) break;

            ans.pb (u);
        }
        sort(all(ans));
        cout << (int)ans.size() << "\n";
        for(int v : ans){
			cout << v << "\n";
		}
        //~ cout << u <<" "<< mn<<"\n";
        //~ cout << par[5] <<" "<< par[2]<<"\n";
    }

    return 0;
}


Measured memory_bytes: 384