from _collections import defaultdict
n,k=map(int,input().split())
root=[[] for i in range(n+10)]

class topological_sort():
    #1-indexed
    #0-indexedはバグる
    def __init__(self,n):
        self.n=n
        self.root=[[] for _ in range(self.n+1)]
        self.deg=[0]*(self.n+1)
    def add(self,a,b):
        #有向辺 a->b の追加
        self.root[a].append(b)
        self.deg[b]+=1
    def const(self):
        #トポロジカルソートした結果を返す
        #不可能なら [] を返す
        deg2=self.deg[:]
        search=[]
        res=[]
        for i in range(1,self.n+1):
            if deg2[i]==0:search.append(i)
        while search:
            now=search.pop()
            res.append(now)
            for next in self.root[now]:
                deg2[next]-=1
                if deg2[next]==0:
                    search.append(next)
        if len(res)==self.n:
            return res
        return []

tp=topological_sort(n)

for i in range(k):
    a,b=map(int,input().split())
    root[a].append(b)
    tp.add(a,b)

tps=tp.const()
if len(tps)==0:
    print(-1)
    exit()

ans=0
dp=[0]*(n+1)
tps.reverse()
for x in tps:
    for y in root[x]:
        dp[x]=max(dp[x],dp[y]+1)
    ans=max(ans,dp[x])

print(ans+1)