/* -*- coding: utf-8 -*-
 *
 * 1660.cc:  No.1660 Matrix Exponentiation - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

const int MAX_N = 100000;

/* typedef */

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

/* global variables */

vi nbrs[MAX_N];
int pns[MAX_N], ds[MAX_N];

/* subroutines */

/* main */

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

  for (int i = 0; i < k; i++) {
    int ri, ci;
    scanf("%d%d", &ri, &ci);
    ri--, ci--;

    nbrs[ri].push_back(ci);
    pns[ci]++;
  }

  int m = 0, maxd = 0;
  qi q;
  
  for (int u = 0; u < n; u++)
    if (pns[u] == 0) ds[u] = maxd = 1, q.push(u), m++;

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

    for (auto v: nbrs[u]) {
      ds[v] = max(ds[v], ds[u] + 1);
      maxd = max(maxd, ds[v]);
      if (--pns[v] == 0) q.push(v), m++;
    }
  }

  printf("%d\n", (m == n) ? maxd: -1);
  return 0;
}