#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <climits>
#include <cfloat>
#include <ctime>
#include <cassert>
#include <map>
#include <utility>
#include <set>
#include <iostream>
#include <memory>
#include <string>
#include <vector>
#include <algorithm>
#include <functional>
#include <sstream>
#include <complex>
#include <stack>
#include <queue>
#include <numeric>
#include <list>
#include <iomanip>
#include <fstream>
#include <bitset>
   
using namespace std;
   
 
#define rep(i, n) for (int i = 0; i < (int)(n); ++i)
#define foreach(it, c) for (__typeof__((c).begin()) it=(c).begin(); it != (c).end(); ++it)
#define rforeach(it, c) for (__typeof__((c).rbegin()) it=(c).rbegin(); it != (c).rend(); ++it)
#define all(c) (c).begin(), (c).end()
#define rall(c) (c).rbegin(), (c).rend()
#define CL(arr, val) memset(arr, val, sizeof(arr))
#define COPY(dest, src) memcpy(dest, src, sizeof(dest))
#define ten(n) ((long long)(1e##n))
#define bin(n) (1ull << (n))
#define erep(i, n) for (int i = 0; i <= (int)(n); ++i)
#define revrep(i, n) for (int i = (n); i >= 0; --i)
#define pb push_back

template <class T> void chmax(T& a, const T& b) { a = max(a, b); }
template <class T> void chmin(T& a, const T& b) { a = min(a, b); }

template <class T> void uniq(T& c) { sort(c.begin(), c.end()); c.erase(unique(c.begin(), c.end()), c.end()); }
 
template <class T> string to_s(const T& a) { ostringstream os; os << a; return os.str(); }
template <class T> T to_T(const string& s) { istringstream is(s); T res; is >> res; return res; }
  
template <class T, class U> ostream& operator<<(ostream& os, pair<T, U>& p) { os << "( " << p.first << ", " << p.second << " )"; return os; }
  
template <class T> void print(T a, int n, const string& deli = " ", int br = 1) { for (int i = 0; i < n; ++i) { cout << a[i]; if (i + 1 != n) cout << deli; } while (br--) cout << endl; }
template <class T> void print(const T& c, const string& deli = " ", int br = 1) { foreach (it, c) { cout << *it; if (++it != c.end()) cout << deli;--it; } while (br--) cout << endl; }
template <class T> void print2d(T a, int w, int h, int width = -1, int br = 1) { for (int i = 0; i < h; ++i) { for (int j = 0; j < w; ++j) {    if (width != -1) cout.width(width); cout << a[i][j] << ' '; } cout << endl; } while (br--) cout << endl; }
   
template <class T> void input(T& a, int n) { for (int i = 0; i < n; ++i) cin >> a[i]; }
template <class T> void input(T* a, int n) { for (int i = 0; i < n; ++i) cin >> a[i]; }

void fix_pre(int n) { cout.setf(ios::fixed, ios::floatfield); cout.precision(10); }
void fast_io() { cin.tie(0); ios::sync_with_stdio(false); }
#define trace(x) (cout << #x << ": " << (x) << endl)
 
bool in_rect(int x, int y, int w, int h) { return 0 <= x && x < w && 0 <= y && y < h; }


typedef long long ll;
typedef pair<int, int> pint;
 
// y(v): v>^<  y(^): ^>v<
const int dx[] = { 0, 1, 0, -1 };
const int dy[] = { 1, 0, -1, 0 };

const double PI = acos(-1.0);



int main()
{
    fast_io();

    int n, m;
    cin >> n >> m;

    vector<int> g[ten(5)];
    map<pint, int> rela_no; // (a, b)
    rep(i, m)
    {
        int a, b;
        cin >> a >> b;
        --a, --b;
        rela_no[pint(a, b)] = i + 1;
        g[a].pb(b);
    }

    // 
    int dp[ten(5)] = {}; // max len path(x -> i)
    vector<int> prev[ten(5)];
    rep(a, n)
    {
        int nd = dp[a] + 1;
        rep(j, g[a].size())
        {
            int b = g[a][j];
            if (nd > dp[b])
            {
                dp[b] = nd;
                prev[b].clear();
            }
            if (nd == dp[b])
                prev[b].pb(a);
        }
    }


    // 
    vector<int> rela[ten(5) + 10]; // (dis) -> rela_no
    int max_len = *max_element(dp, dp + n);
    bool visit[ten(5)] = {};
    queue<int> q;
    rep(b, n)
    {
        if (dp[b] == max_len)
        {
            q.push(b);
            visit[b] = true;
        }
    }

    while (!q.empty())
    {
        int b = q.front();
        q.pop();
        
        rep(i, prev[b].size())
        {
            int a = prev[b][i];
            rela[dp[b]].pb(rela_no[pint(a, b)]);

            if (!visit[a])
            {
                q.push(a);
                visit[a] = true;
            }
        }
    }

    vector<int> res;
    erep(i, n)
        if (rela[i].size() == 1)
            res.pb(rela[i][0]);
    if (res.empty())
        cout << -1 << endl;
    else
    {
        sort(all(res));
        rep(i, res.size())
            cout << res[i] << endl;
    }
}

Measured memory_bytes: 25848