import sys
input = sys.stdin.readline
from collections import *
def topsort(G):
    Q = deque()
    N = len(G)
    count = [0] * N

    for u in range(N):
        for v in G[u]:
            count[v] += 1

    for u in range(N):
        if count[u] == 0:
            Q.append(u)
    anslst = []
    while Q:
        u = Q.pop()
        anslst.append(u)
        for v in G[u]:
            count[v] -= 1
            if count[v] == 0:
                Q.append(v)
    return anslst


N, K = map(int, input().split())   
G = [[] for i in range(N)]
for i in range(K):
    a, b = map(int, input().split())
    a, b = a - 1, b - 1
    if a == b:
        print(-1)
        exit()
    G[a].append(b)
    
A = topsort(G)
if len(A) != N:
    print(-1)
    exit()
    
dp = [0] * N
for a in A:
    for u in G[a]:
        dp[u] = max(dp[u], dp[a] + 1)
        
print(max(dp) + 1)