from collections import deque

class DetectDAG:
	# REQUIRED
	# from collections import deque
	def __init__(self, n):
		self.n = n
		self.ikeru = [[] for i in range(n)]
		self.deg = [0] * n
	
	def add_edge(self, x, y):
		self.ikeru[x].append(y)
		self.deg[y] += 1

	def isdag(self):
		mada = deque()
		for i in range(self.n):
			if self.deg[i] == 0:
				mada.append(i)
		while mada:
			i = mada.popleft()
			for j in self.ikeru[i]:
				self.deg[j] -= 1
				if self.deg[j] == 0:
					mada.append(j)
		for i in self.deg:
			if i > 0: return False
		return True

n, q = map(int,input().split())
x = [0] * q
y = [0] * q
for i in range(q):
	a, b = map(int,input().split())
	x[i] = a-1
	y[i] = b-1

suki = q
kirai = -1
while suki - kirai > 1:
	targ = (suki + kirai) // 2
	g = DetectDAG(n)
	for i in range(targ):
		g.add_edge(x[i], y[i])
	if not g.isdag():
		suki = targ
	else:
		kirai = targ

if suki == q: print(-1)
else: print(suki)