n, m = map(int, input().split())
G = [[] for i in range(n)]
d = [0] * n
for _ in range(m):
    u, v = map(int, input().split())
    G[u - 1].append(v - 1)
    d[v - 1] += 1
def topological_sort(G):
    s = []
    for i in range(n):
        if d[i] == 0: s.append(i)
    ans = []
    while s:
        u = s.pop()
        ans.append(u)
        for v in G[u]:
            d[v] -= 1
            if d[v] == 0: s.append(v)
    if len(ans) != n: return -1
    return ans
if topological_sort(G) == -1: exit(print(-1)) 
visit = [False] * n
dp = [0] * n
def dfs(now):
    if visit[now]:
        return dp[now]
    visit[now] = True
    res = 0
    for i in G[now]:
        res = max(res, dfs(i) + 1)
    dp[now] = res
    return res
ans = 0
for i in range(n):
    ans = max(ans, dfs(i))
print(ans + 1)