#include <iostream>
#include <vector>
#include <string>
#include <set>
#include <map>
#include <queue>

using namespace std;
#define pll pair<long,pair<long,long>>

long res = -1;

long rooms, arrows;
long temp, temp2;
vector<set<long>> a;

set<long> used;

bool dfs(long start, set<long> visited = {}) {
    visited.insert(start);

    set<long> connections = a[start];

    for (auto connection : connections) {
        if (visited.size() >= rooms-1) {
            return true;
        }

        if (visited.find(connection) == visited.end()) {
            if (dfs(connection, visited)) return true;
        }
    }
    return false;
}

void main()
{
    cin >> rooms >> arrows;
    a.resize(rooms);

    for (int i = 0; i < arrows; i++) {
        cin >> temp >> temp2;
        a[temp - 1].insert(temp2 - 1);
    }

    for (int i = 0; i < rooms; i++) {
        if (dfs(i)) res = i + 1;
    }

    cout << res;
}