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 i, (a, b) in enumerate(edges):
    deg[b] += 1
    G[a].append((i, b))

deg_is_zero = set(i for i, x in enumerate(deg) if not x)
d = deque(deg_is_zero)
visited_edge = [False] * Q


def dfs(d):
    while d:
        v = d.popleft()
        for i, x in G[v]:
            if visited_edge[i]:
                continue
            visited_edge[i] = True
            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):
    if visited_edge[i - 1]:
        continue
    b = edges[i - 1][1]
    deg[b] -= 1
    visited_edge[i - 1] = True
    if not deg[b]:
        deg_is_zero.add(b)
        dfs(deque([b]))
        if len(deg_is_zero) == N:
            print(i)
            exit()