import sys
sys.setrecursionlimit(10**6)

def scc(N, G, RG):
    order = []
    used = [0]*N
    group = [None]*N
    def dfs(s):
        used[s] = 1
        for t in G[s]:
            if not used[t]:
                dfs(t)
        order.append(s)
    def rdfs(s, col):
        group[s] = col
        used[s] = 1
        for t in RG[s]:
            if not used[t]:
                rdfs(t, col)
    for i in range(N):
        if not used[i]:
            dfs(i)
    used = [0]*N
    label = 0
    for s in reversed(order):
        if not used[s]:
            rdfs(s, label)
            label += 1
    return label, group

def construct(N, G, label, group):
    G0 = [set() for i in range(label)]
    GP = [[] for i in range(label)]
    for v in range(N):
        lbs = group[v]
        for w in G[v]:
            lbt = group[w]
            if lbs == lbt:
                continue
            G0[lbs].add(lbt)
        GP[lbs].append(v)
    return G0, GP

n,m=map(int,input().split())
edge=[[] for i in range(n)]
redge=[[] for i in range(n)]
for _ in range(m):
  a,b=map(lambda x:int(x)-1,input().split())
  if a==b:
    print(-1)
    exit()
  edge[a].append(b)
  redge[b].append(a)

label,group=scc(n,edge,redge)
if label!=n:
  print(-1)
  exit()

dp=[1]*n
G0,GP=construct(n,edge,n,group)
for i in range(n):
  for j in G0[i]:
    dp[j]=max(dp[j],dp[i]+1)
print(max(dp))