import sys
from collections import deque

input = sys.stdin.buffer.readline


def solve(N: int, K: int, G: list[list[int]]):
    deg = [0] * N
    for i in range(N):
        for j in G[i]:
            deg[j] += 1

    q = deque(v for v in range(N) if deg[v] == 0)
    visited = [False] * N
    max_chain = [1] * N
    while q:
        v = q.popleft()
        if visited[v]:
            continue
        visited[v] = True
        for t in G[v]:
            deg[t] -= 1
            max_chain[t] = max(max_chain[t], max_chain[v] + 1)
            if deg[t] == 0:
                q.append(t)
    if max(deg) == 0:
        return max(max_chain)
    else:
        return -1


N, K = map(int, input().split())
G = [[] for _ in range(N)]
deg = [0] * N
for _ in range(K):
    r, c = map(lambda x: int(x) - 1, input().split())
    G[r].append(c)

print(solve(N, K, G))