#include <iostream>
#include <vector>
#include <queue>
using namespace std;

int vis[100010];
int visr[100010];

int dfs(int fst, int nw, int dist, vector<int>* A){
  int mx = 0;
  for(int i = 0; A[nw].size() > i; i++){
    if(vis[A[nw][i]] == fst){
      return -1;
    }
    vis[A[nw][i]] = visr[A[nw][i]] = fst;
    mx = max(mx, dfs(fst, A[nw][i], dist+1, A));
    vis[A[nw][i]] = -1;
  }
  mx = max(mx, dist);
  return mx;
}

int main(){
  int n,m;cin>>n>>m;
  vector<int> A[n];
  vector<int> B(n);
  for(int i = 0; n > i; i++)vis[i] = visr[i] = -1;
  for(int i = 0; m > i; i++){
    int u,v;cin>>u>>v;
    u--;v--;
    A[u].push_back(v);
    B[v]++;
  }
  int mx = 0;
  for(int i = 0; n > i; i++){
    if(!B[i]){
      vis[i] = visr[i] = i;
      mx = max(mx, dfs(i, i, 0, A));
      vis[i] = -1;
    }
  }
  for(int i = 0; n > i; i++){
    if(visr[i] == -1){
      cout << -1 << endl;
      return 0;
    }
  }
  cout << mx+1 << endl;

}