N,M=map(int,input().split())
G=[[] for i in range(N)]
for i in range(M):
  a,b=map(int,input().split())
  a-=1
  b-=1
  if a==b:
    print(-1)
    exit()
  G[a].append(b)
def SCC(G):
  D=[]
  C=[len(G)-1]
  U=[1]*len(G)
  def DFS(x):
    if U[x]:
      U[x]=0
      for i in range(len(G[x])):
        DFS(G[x][i])
      D.append(x)
  
  for i in range(len(G)):
    if U[i]:
      DFS(i)
  GR=[[] for i in range(len(G))]
  for i in range(len(G)):
    for j in range(len(G[i])):
      GR[G[i][j]].append(i)
  R=[]
  U=[1]*len(G)
  def DFSR(x):
    if U[x]:
      R[-1].append(x)
      U[x]=0
      for i in range(len(GR[x])):
        DFSR(GR[x][i])
  
  for i in range(len(G)-1,-1,-1):
    if U[D[i]]:
      R.append([])
      DFSR(D[i])
  return R

X=SCC(G)
if max([len(X[i]) for i in range(len(X))])>1:
  print(-1)
  exit()

DP=[-1]*N
def f(x):
  if DP[x]!=-1:
    return DP[x]
  DP[x]=0
  for i in range(len(G[x])):
    DP[x]=max(DP[x],f(G[x][i])+1)
  return DP[x]
 
print(max([f(i)for i in range(N)])+1)