import sys,random,bisect
from collections import deque,defaultdict
from heapq import heapify,heappop,heappush
from itertools import permutations
from math import log,gcd

input = lambda :sys.stdin.readline().rstrip()
mi = lambda :map(int,input().split())
li = lambda :list(mi())

N,M = mi()
E = []
for _ in range(M):
    u,v = mi()
    E.append((u-1,v-1))

def cond(k):
    edge = [[] for v in range(N)]
    deg = [0] * N
    for i in range(k):
        u,v = E[i]
        edge[u].append(v)
        deg[v] += 1
    
    deq = deque([v for v in range(N) if deg[v]==0])
    while deq:
        v = deq.popleft()
        for nv in edge[v]:
            deg[nv] -= 1
            if deg[nv]==0:
                deq.append(nv)
    
    return any(deg[v] for v in range(N))

ok = M+1
ng = 0
while ok-ng>1:
    mid = (ok+ng)//2
    if cond(mid):
        ok = mid
    else:
        ng = mid

if ok==M+1:
    print(-1)
else:
    print(ok)