#include <bits/stdc++.h>
using namespace std;


#define li          long long int
#define rep(i,to)   for(li i=0;i<((li)(to));i++)
#define repp(i,start,to)    for(li i=(li)(start);i<((li)(to));i++)
#define pb          push_back
#define sz(v)       ((li)(v).size())
#define bgn(v)      ((v).begin())
#define eend(v)     ((v).end())
#define allof(v)    (v).begin(), (v).end()
#define dodp(v,n)       memset(v,(li)n,sizeof(v))
#define bit(n)      (1ll<<(li)(n))
#define mp(a,b)     make_pair(a,b)
#define rin rep(i,n)
#define EPS 1e-12
#define ETOL 1e-8
#define MOD 1000000007
typedef pair<li, li> PI;

#define INF bit(60)

#define DBGP 1


#define idp if(DBGP)
#define F first
#define S second
#define p2(a,b)     idp cout<<a<<"\t"<<b<<endl
#define p3(a,b,c)       idp cout<<a<<"\t"<<b<<"\t"<<c<<endl
#define p4(a,b,c,d)     idp cout<<a<<"\t"<<b<<"\t"<<c<<"\t"<<d<<endl
#define p5(a,b,c,d,e)       idp cout<<a<<"\t"<<b<<"\t"<<c<<"\t"<<d<<"\t"<<e<<endl
#define p6(a,b,c,d,e,f)     idp cout<<a<<"\t"<<b<<"\t"<<c<<"\t"<<d<<"\t"<<e<<"\t"<<f<<endl
#define p7(a,b,c,d,e,f,g)       idp cout<<a<<"\t"<<b<<"\t"<<c<<"\t"<<d<<"\t"<<e<<"\t"<<f<<"\t"<<g<<endl
#define p8(a,b,c,d,e,f,g,h)     idp cout<<a<<"\t"<<b<<"\t"<<c<<"\t"<<d<<"\t"<<e<<"\t"<<f<<"\t"<<g<<"\t"<<h<<endl
#define p9(a,b,c,d,e,f,g,h,i)       idp cout<<a<<"\t"<<b<<"\t"<<c<<"\t"<<d<<"\t"<<e<<"\t"<<f<<"\t"<<g<<"\t"<<h<<"\t"<<i<<endl
#define p10(a,b,c,d,e,f,g,h,i,j)        idp cout<<a<<"\t"<<b<<"\t"<<c<<"\t"<<d<<"\t"<<e<<"\t"<<f<<"\t"<<g<<"\t"<<h<<"\t"<<i<<"\t"<<j<<endl
#define foreach(it,v)   for(__typeof((v).begin()) it=(v).begin(); it!=(v).end(); ++it)
#define p2p(x)      idp p2((x).F, (x).S)
#define dump(x,n)   idp{rep(i,n){cout<<x[i]<<" ";}puts("");}
#define dump2(x,n)  idp{rep(i,n){cout<<"["<<x[i].F<<" , "<<x[i].S<<"] ";}puts("");}
#define dumpi(x)    idp{foreach(it, x){cout<<(*it)<<" ";}puts("");}
#define dumpi2(x)   idp{foreach(it, x){cout<<"["<<(it)->F<<" , "<<(it)->S<<"] ";}puts("");}

#define read2d(a,w,h)   rep(i,h)rep(j,w)cin>>a[i][j]
#define dump2d(a,w,h)   rep(i,h){rep(j,w)cout<<a[i][j]<<" ";puts("");}

vector<li> edge[2020];
li n, m;

inline vector<li> check_loop(li root) {
    vector<li> from(n, -1);
    vector<li> cost(n, INF);
    vector<li> res;

    queue<pair<li, PI>> q;
    q.push({ 0, {root, -1}});
    while (!q.empty()) {
        li now_c = q.front().F;
        li now = q.front().S.F;
        li prev = q.front().S.S;
        //p3(now, prev, now_c);
        q.pop();
        if (prev >= 0 && now == root) {
            from[now] = prev;
            break;
        }
        if (cost[now] <= now_c) {
            continue;
        }

        cost[now] = now_c;
        from[now] = prev;
        for (auto && next : edge[now]) {
            q.push({now_c + 1, {next, now}});
        }
    }
    if (from[root] < 0) {
        return res;
    }
    li now = root;
    while (true) {
        //p2(now, from[now]);
        res.pb(now);
        now = from[now];
        if (now == root)break;
    }
    reverse(allof(res));
    return res;
}

int main() {
    cin >> n >> m;
    rep(i, m) {
        li a, b;
        cin >> a >> b;
        a--;
        b--;
        edge[a].pb(b);
    }

    vector<li> resv;
    rin{
        auto v = check_loop(i);
        if (!v.empty()) {
            if (resv.empty() || sz(resv) > sz(v)) {
                resv = v;
            }
        }
    }
    if (resv.empty()) {
        puts("-1");
    } else {
        cout << sz(resv) << endl;
        for (auto && v : resv) {
            cout << v + 1 << endl;
        }
    }

    return 0;
}

Measured memory_bytes: 384