import sys
from collections import deque

input = sys.stdin.buffer.readline

N, Q = map(int, input().split())
edges = tuple(tuple(int(x) - 1 for x in input().split()) for _ in range(Q))
deg = [0] * N
G = [[] for _ in range(N)]
for a, b in edges:
    deg[b] += 1
    G[a].append(b)

deg_is_zero = set(i for i, x in enumerate(deg) if not x)
d = deque(deg_is_zero)


def dfs(d):
    while d:
        v = d.popleft()
        for x in G[v]:
            deg[x] -= 1
            if not deg[x]:
                d.append(x)
                deg_is_zero.add(x)


dfs(d)

if len(deg_is_zero) == N:
    print(-1)

for i in range(Q, 0, -1):
    b = edges[i - 1][1]
    deg[b] -= 1
    if not deg[b]:
        deg_is_zero.add(b)
        dfs(deque([b]))
        if len(deg_is_zero) == N:
            print(i)