"""
1865:
二分探索でok

"""

import sys
from sys import stdin
from collections import deque

N,Q = map(int,stdin.readline().split())

AB = []
for i in range(Q):
    A,B = map(int,stdin.readline().split())
    A -= 1
    B -= 1
    AB.append( (A,B) )

L = 0
R = Q+1

while R-L != 1:

    M = (L+R)//2
    lis = [ [] for i in range(N) ]
    inlis = [0] * N

    for i in range(M):
        a,b = AB[i]
        lis[a].append(b)
        inlis[b] += 1

    q = deque([])

    for i in range(N):
        if inlis[i] == 0:
            q.append(i)

    end = 0
    while q:
        v = q.popleft()
        end += 1
        for nex in lis[v]:
            inlis[nex] -= 1
            if inlis[nex] == 0:
                q.append(nex)

    if end == N:
        L = M
    else:
        R = M

if R == Q+1:
    R = -1
print (R)