from collections import deque, defaultdict
n, Q = map(int,input().split())

query = []
for i in range(Q):
	a, b = map(int,input().split())
	query.append((a-1, b-1))

def check_cycle(r):
	ins = [0 for i in range(n)]
	outs = [[] for i in range(n)]
	for a, b in query[:r]:
		ins[b] += 1
		outs[a].append(b)
	res = []
	mada = deque()
	for i in range(n):
		if ins[i] == 0:
			mada.append(i)
	while len(mada) > 0:
		i = mada.popleft()
		res.append(i)
		for j in outs[i]:
			ins[j] -= 1
			if ins[j] == 0:
				mada.append(j)
	if len(res) != n:
		return True
	return False

suki = Q + 1
kirai = 0

while suki - kirai > 1:
	targ = (suki + kirai) // 2
	if check_cycle(targ):
		suki = targ
	else:
		kirai = targ

if suki > Q:
	print(-1)
else:
	print(suki)