from collections import defaultdict
import sys


N, K = map(int, input().split())
graph = defaultdict(list)
for _ in range(K):
    r, c = map(lambda x:int(x)-1, input().split())
    graph[r].append(c)

memo = [0] * (N+1)
seen = [False] * (N+1)
sys.setrecursionlimit(1000000)
def dfs(v):
    if memo[v]:
        return memo[v]
    seen[v] = True
    res = 1
    for next_v in graph[v]:
        if seen[next_v] and not memo[next_v]:
            print(-1)
            sys.exit()
        res = max(res, dfs(next_v)+1)
    memo[v] = res
    return res

for i in range(N):
    if memo[i]:
        continue
    dfs(i)
print(max(memo))