import sys
sys.setrecursionlimit(10 ** 6)

def dfs(v):
	visit[v] = 1
	for u in G[v]:
		if visit[u] == 1 or (not visit[u] and dfs(u)): return 1
	visit[v] = 2
	return 0

n, q = map(int, input().split())
E = []
for _ in range(q):
	a, b = map(int, input().split())
	a -= 1; b -= 1
	E.append((a, b))
l, r = -1, q + 1
while r - l > 1:
	m = (l + r) // 2
	G = [[] for _ in range(n)]
	for a, b in E[:m]:
		G[a].append(b)
	visit = [0] * n
	f = False
	for i in range(n):
		if not visit[i]: f |= dfs(i)
	if f: r = m
	else: l = m
print(r if r <= q else -1)