/*
ID: hepic
PROG: sabotage
LANG: C++11
*/
#include <bits/stdc++.h>

#define FOR(i, a, b) for(auto i=a; i<=b; ++i)
#define REP(i, a, b) for(auto i=a; i<b; ++i)
#define FORI(i, a, b) for(auto i=a; i!=b+1-2*(a>b); i+=1-2*(a>b))
#define REPI(i, a, b) for(auto i=a-(a>b); i!=b-(a>b); i+=1-2*(a>b))
#define ALL(v) v.begin(),v.end()
#define mp(a, b) make_pair(a, b)
#define pb(a) push_back(a)
#define pf(a) push_front(a)
#define eb(a, b) emplace_back(a, b)
#define fir first
#define sec second
#define what_is(x) cout<<#x<<" is "<<x<<endl;
#define type(x) typeid(x).name()
#define ms(arr, val) memset(arr, val, sizeof(arr))
#define min3(a,b,c) min(min(a,b),c)
#define max3(a,b,c) max(max(a,b),c)
#define PI acos(-1)
#define open_read freopen("disrupt.in", "r", stdin)
#define open_write freopen("disrupt.out", "w", stdout)

using namespace std;

typedef long long LL;
typedef long double LD;
typedef unsigned long long ULL;
typedef pair<double, double> PDD;
typedef pair<int, int> PII;
typedef pair<int, PII> PIPII;
typedef pair<PII, PII> PPIIPII;
typedef pair<LL, LL> PLL;
typedef pair<ULL, ULL> PUU;
typedef pair<LL, PLL> PLPLL;
typedef pair<PLL, PLL> PPLLPLL;
typedef pair<int, LL> PIL;
typedef pair<LL, int> PLI;


template<typename T, typename T1>
ostream& operator << (ostream &out, pair<T, T1> obj) {
    out << "(" << obj.first << ", " << obj.second << ")";
    return out;
}


template<typename T, typename T1>
ostream& operator << (ostream &out, map<T, T1> cont) {
    typename map<T, T1>::const_iterator itr = cont.begin();
    typename map<T, T1>::const_iterator ends = cont.end();

    for(; itr != ends; ++itr) {
        out << *itr << " ";
    }

    out<<endl;
    return out;
}


template<typename T>
ostream& operator << (ostream &out, set<T> cont) {
    typename set<T>::const_iterator itr = cont.begin();
    typename set<T>::const_iterator ends = cont.end();
    
    for(; itr != ends; ++itr) {
        out << *itr << " ";
    }

    out << endl;
    return out;
}


template<typename T>
ostream& operator << (ostream &out, multiset<T> cont) {
    typename multiset<T>::const_iterator itr = cont.begin();
    typename multiset<T>::const_iterator ends = cont.end();

    for (; itr != ends; ++itr) {
        out << *itr << " ";
    }

    out << endl;
    return out;
}


template<typename T, template<typename ELEM, typename ALLOC=allocator<ELEM>> class CONT>
ostream& operator << (ostream &out, CONT<T> cont) {
    typename CONT<T>::const_iterator itr = cont.begin();
    typename CONT<T>::const_iterator ends = cont.end();

    for (; itr != ends; ++itr) {
        out << *itr << " ";
    }

    out << endl;
    return out;
}


template<typename T, unsigned int N, typename CTy, typename CTr>
typename enable_if<!is_same<T, char>::value, basic_ostream<CTy, CTr> &>::type
operator << (basic_ostream<CTy, CTr> &out, const T (&arr)[N]) {
    REP (i, 0, N) {
        out << arr[i] << " ";
    }

    out << endl;
    return out;
}


template<typename T>
T GCD(T a, T b) {
    T min_v = min(a, b);
    T max_v = max(a, b);

    while (min_v) {
        T temp = max_v % min_v;
        max_v = min_v;
        min_v = temp;
    }

    return max_v;
}


template<typename T>
T LCM(T a, T b) {
    return (a*b) / gcd(a, b);
}


template<typename T>
T fastExpPow(T base, T exp, T mod) {
    T res = 1;

    while (exp) {
        if (exp&1) {
            res *= base;
            res %= mod;
        }

        exp >>= 1;
        base *= base;
        base %= mod;
    }

    return res % mod;
}

/*#################################################################################################################
###################################################################################################################
###################################################################################################################
#################################################################################################################*/

#define SIZE 1010

int N, M, root;
int parent[SIZE];
bool visited[SIZE], inside[SIZE];
bool check;
vector<int> graph[SIZE], result;


void dfs(int node) {
    visited[node] = inside[node] = true;
    vector<int> goodNeigh;
    int cnt = 0;

    for (auto connNode: graph[node]) {
        if (inside[connNode]) {
            ++cnt;
        }

        if (!visited[connNode]) {
            goodNeigh.pb(connNode);
        }

        visited[connNode] = true;
    }
    
    if (cnt >= 1) {
        if (cnt == 1) {
            for (auto connNode: graph[node]) {
                if (inside[connNode] && connNode == root) {
                    parent[connNode] = node;
                    check = true;
                }
            }
        }

        return;
    }
    
    for (auto connNode: goodNeigh) {
        if (!check) {
            parent[connNode] = node;
            dfs(connNode);
            inside[connNode] = false;        
        }
    }
}

void subGraph(int node) {
    int temp = node;

    do {
        result.pb(parent[node]); 
        node = parent[node];
    } while (node != temp);
}



int main() {
    scanf("%d%d", &N, &M);

    REP (i, 0, M) {
        int A, B;
        scanf("%d%d", &A, &B);
        graph[A].pb(B);
    }
    
    FOR (node, 1, N) {
        ms(visited, false);
        ms(inside, false);
        ms(parent, -1);
        check = false;
        
        root = node;
        dfs(node);

        if (check) {
            subGraph(node);
            printf("%d\n", (int)result.size());

            for (auto elem: result) {
                printf("%d\n", elem);
            }

            return 0;
        }
    }
    
    printf("-1\n");
    return 0;
}

Measured memory_bytes: 384