from collections import deque, defaultdict

n, q = map(int, input().split())
Q = []
for _ in range(q):
    u, v = map(int, input().split())
    u -= 1
    v -= 1
    Q.append((u, v))

def check_cycle(r):
    INcnt = defaultdict(int)
    # INcnt[v]:到着先がvである回数
    OUTs = defaultdict(list)
    # OUTs[u]:出発点がuであるときの到着先リスト
    for u, v in Q[:r]:
        INcnt[v] += 1
        OUTs[u].append(v)
    Topo = []
    Que = deque([i for i in range(n) if INcnt[i] == 0])
    while len(Que) != 0:
        v = Que.popleft()
        Topo.append(v)
        for v2 in OUTs[v]:
            INcnt[v2] -= 1
            if INcnt[v2] == 0:
                Que.append(v2)
    return len(Topo) != n

left = 0
right = q + 1
while right - left > 1:
    mid = (left + right) // 2
    if check_cycle(mid):
        right = mid
    else:
        left = mid
if right > q:
    print(-1)
else:
    print(right)