N,K=map(int,input().split())
graph=[[] for x in range(N)]
degree=[0]*N
for k in range(K):
    r,c=map(int,input().split())
    r-=1;c-=1
    graph[r].append(c)
    degree[c]+=1
queue=[]
inf=1<<30
dp=[-inf]*N
for x in range(N):
    if degree[x]==0:
        queue.append(x)
        dp[x]=0
while queue:
    x=queue.pop()
    for y in graph[x]:
        dp[y]=max(dp[y],dp[x]+1)
        degree[y]-=1
        if degree[y]==0:
            queue.append(y)
if sum(degree):
    ans=-1
else:
    ans=max(dp)+1
print(ans)