#include <iostream>
#include <cmath>
#include <string>
#include <vector>
#include <algorithm>
#include <utility>
#include <tuple>
#include <cstdint>
#include <cstdio>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <deque>
#include <unordered_map>
#include <unordered_set>
#include <bitset>
#include <cctype>
#include <climits>
#include <functional>
#include <cassert>
#include <numeric>
#define rep(i, n) for(int i = 0; i < (n); i++)
#define per(i, n) for(int i = (n) - 1; i >= 0; i--)
using ll = long long;
#define vi vector<int>
#define vvi vector<vi>
#define vl vector<ll>
#define pii pair<int, int>
#define pll pair<ll, ll>
#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define mod 1000000007
using namespace std;
template<class T, class U>
bool chmax(T &a, const U &b){ return a < b ? (a = b, 1) : 0; }
template<class T, class U>
bool chmin(T &a, const U &b){ return a > b ? (a = b, 1) : 0; }
template <class T>
struct Edge {
  int from,to;
  T cost;
  int idx;
  Edge(){};
  Edge(int f, int t, T c=1, int i=-1) : from(f), to(t), cost(c), idx(i){}
  Edge(int t) : to(t), from(-1), cost(1), idx(-1){}
  operator int() const{ return to; }
  bool operator<(const Edge &e){ return cost < e.cost; }
};
template <class T>
struct Graph : vector<vector<Edge<T>>> {
  Graph(){}
  Graph(const int &n) : vector<vector<Edge<T>>>(n){}
  void add_edge(int a, int b, T c=1, int i=-1){
    (*this)[a].push_back({ a, b, c, i });
  }
};
using graph = Graph<int>;
namespace cycle {
  graph g;
  vector<int> used;
  vector<Edge<int>> prev, cyc;
  bool dfs(int pos){
    used[pos] = 1;
    for(auto &x : g[pos]){
      if(!used[x]){
        prev[x] = x;
        if(dfs(x)) return true;
      }else if(used[x] == 1){
        int cur = pos;
        cyc.push_back(x);
        while(cur != x){
          cyc.push_back(prev[cur]);
          cur = prev[cur].from;
        }
        return true;
      }
    }
    used[pos] = 2;
    return false;
  }
  vector<Edge<int>> detect(const graph &root){
    g = root;
    int n = g.size();
    used.assign(n, 0);
    prev.assign(n, -1);
    for(int i = 0; i < n; i++){
      if(!used[i] && dfs(i)){
        reverse(cyc.begin(), cyc.end());
        return cyc;
      }
    }
    return {};
  }
};
int main(){
  int n,q;
  cin >> n >> q;
  vector<Edge<int>> es(q);
  rep(i, q){
    cin >> es[i].from >> es[i].to;
    es[i].from--; es[i].to--;
  }
  int left = 0, right = q;
  while(left + 1 < right){
    int mid = (left + right) / 2;
    graph root(n);
    rep(i, mid) root.add_edge(es[i].from, es[i].to);
    auto res = cycle::detect(root);
    if(res.empty()) left = mid;
    else right = mid;
  }
  if(right == q) cout << "-1\n";
  else cout << right << "\n";
}