/* -*- coding: utf-8 -*-
 *
 * 1865.cc:  No.1865 Make Cycle - yukicoder
 */

#include<cstdio>
#include<vector>
#include<queue>
#include<algorithm>
 
using namespace std;

/* constant */

const int MAX_N = 100000;
const int MAX_QN = 100000;

/* typedef */

typedef vector<int> vi;
typedef queue<int> qi;
typedef pair<int,int> pii;

/* global variables */

vi nbrs[MAX_N];
pii es[MAX_QN];
int pns[MAX_N];

/* subroutines */

bool check(int n, int m) {
  for (int i = 0; i < n; i++)
    nbrs[i].clear(), pns[i] = 0;

  for (int i = 0; i < m; i++) {
    int u = es[i].first, v = es[i].second;
    nbrs[u].push_back(v);
    pns[v]++;
  }

  qi q;
  for (int u = 0; u < n; u++)
    if (pns[u] == 0) q.push(u);

  int c = 0;
  while (! q.empty()) {
    int u = q.front(); q.pop();
    c++;

    for (auto v: nbrs[u])
      if (--pns[v] == 0) q.push(v);
  }

  return (c == n);
}

/* main */

int main() {
  int n, qn;
  scanf("%d%d", &n, &qn);

  for (int i = 0; i < qn; i++) {
    int u, v;
    scanf("%d%d", &u, &v);
    u--, v--;
    es[i] = pii(u, v);
  }

  int m0 = 0, m1 = qn + 1;
  while (m0 + 1 < m1) {
    int m = (m0 + m1) / 2;
    if (check(n, m)) m0 = m;
    else m1 = m;
  }

  printf("%d\n", (m1 <= qn) ? m1 : -1);
  return 0;
}