from collections import deque

def scc(x,y,n):
  res=[-1]*n
  p=0
  a=[0]*n;b=[0]*n
  for i in range(n):
    if b[i]:
      continue
    b[i]=1
    f=deque([(i,0)])
    while f:
      q,w=f.pop()
      if len(x[q])>w:
        f.append((q,w+1))
        if b[x[q][w]]:
          continue
        f.append((x[q][w],0))
        b[x[q][w]]=1
      else:
        a[p]=q;p+=1
  for i in reversed(a):
    if res[i]!=-1:
      continue
    f=deque([i]);res[i]=i
    while f:
      q=f.pop()
      for j in y[q]:
        if res[j]==-1:
          f.append(j);res[j]=i
  return res
        
n,q=map(int,input().split())
v=[tuple(map(lambda x:int(x)-1,input().split())) for i in range(q)]
a,b=0,q
while b-a>1:
  m=(a+b)//2
  x=[[] for i in range(n)]
  y=[[] for i in range(n)]
  for i in range(m+1):
    x[v[i][0]].append(v[i][1])
    y[v[i][1]].append(v[i][0])
  c=scc(x,y,n)
  for i in range(n):
    if i!=c[i]:
      b=m;break
  else :
    a=m
if b==q:
  b=-2
print(b+1)