import sys
from collections import deque
input = lambda: sys.stdin.readline().rstrip()

def topological_sort(G: list) -> list:
  "Return topological_sort. / O(|V|+|E|)"
  # 0-indexed
  # len(toposo) != n: 閉路が存在
  n = len(G)
  in_cnt = [0] * n
  outs = [[] for _ in range(n)]
  for v in range(n):
    for x in G[v]:
      in_cnt[x] += 1
      outs[v].append(x)
  res = []
  todo = deque([i for i in range(n) if in_cnt[i] == 0])
  while todo:
    v = todo.popleft()
    res.append(v)
    for x in outs[v]:
      in_cnt[x] -= 1
      if in_cnt[x] == 0:
        todo.append(x)
  return res

#  -----------------------  #

n, q = map(int, input().split())
AB = [list(map(lambda x: int(x)-1, input().split())) for _ in range(q)]

def isok(mid):
  G = [[] for _ in range(n)]
  for i in range(mid):
    a = AB[i][0]
    b = AB[i][1]
    G[a].append(b)
  toposo = topological_sort(G)
  if len(toposo) != n:
    return True
  else:
    return False

ok, ng = q+1, -1
while ok - ng > 1:
  mid = (ok + ng) // 2
  if isok(mid):
    ok = mid
  else:
    ng = mid
if ok == q+1:
  print(-1)
else:
  print(ok)