from collections import deque

n,k=map(int,input().split())
x=[0]*n
v=[[] for i in range(n)]
for i in range(k):
    a,b=map(int,input().split());a-=1;b-=1
    v[b].append(a);x[a]+=1
f=deque();dp=[0]*n
for i in range(n):
    if not x[i]:
        f.append(i)
while f:
    q=f.pop()
    for i in v[q]:
        dp[i]=max(dp[i],dp[q]+1)
        x[i]-=1
        if not x[i]:
            f.append(i)
if max(x):
	print(-1);exit()
print(max(dp)+1)