import sys
import pypyjit
pypyjit.set_param('max_unroll_recursion=-1')
sys.setrecursionlimit(200000)



n, m = map(int, input().split())

ikeru = [[] for i in range(n)]
ikeruinv = [[] for i in range(n)]
for j in range(m):
	a, b = map(int, input().split())
	if a == b:
		continue
	ikeru[a-1].append((b-1, j + 1))
	ikeruinv[b-1].append((a-1, j + 1))

def dfs(a):
	global ichi
	if tansaku[a] == 1:
		return
	tansaku[a] = 1
	for i, v in ikeru[a]:
		if v <= targ:
			if tansaku[i] == 0:
				dfs(i)
	dat[a] = ichi
	ichi += 1
	return


def dfsinv(a):
	global mode
	if tansakuinv[a] == 1:
		return
	tansakuinv[a] = 1
	for i, v in ikeruinv[a]:
		if v <= targ:
			if tansakuinv[i] == 0:
				mode = 0
				break
	return

suki = m+3
kirai = -1

while suki - kirai > 1:
	targ = (suki + kirai) // 2
	
	mode = 1
	tansaku = [0 for i in range(n)]
	tansakuinv = [0 for i in range(n)]
	dat = [0 for i in range(n)]
	ichi = 0
	
	for i in range(n):
		dfs(i)
	
	u = [(i, dat[i]) for i in range(n)]
	u.sort(key=lambda x:-x[1])
	
	for i, d in u:
		dfsinv(i)
	#print(mode, targ)
	
	if mode:
		kirai = targ
	else:
		suki = targ

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