import sys
from collections import deque

def main():
    N, K = map(int, sys.stdin.readline().split())
    if K == 0:
        print(1)
        return
    
    adj = [[] for _ in range(N + 1)]
    in_degree = [0] * (N + 1)
    
    for _ in range(K):
        r, c = map(int, sys.stdin.readline().split())
        adj[r].append(c)
        in_degree[c] += 1
    
    q = deque()
    order = []
    for i in range(1, N + 1):
        if in_degree[i] == 0:
            q.append(i)
    
    processed = 0
    while q:
        u = q.popleft()
        order.append(u)
        processed += 1
        for v in adj[u]:
            in_degree[v] -= 1
            if in_degree[v] == 0:
                q.append(v)
    
    if processed != N:
        print(-1)
        return
    
    max_len = 0
    dp = [0] * (N + 1)
    for u in order:
        for v in adj[u]:
            if dp[u] + 1 > dp[v]:
                dp[v] = dp[u] + 1
                if dp[v] > max_len:
                    max_len = dp[v]
    
    print(max_len + 1)

if __name__ == "__main__":
    main()