import collections
def is_ok(m):
    L = [[] for _ in range(N+1)]
    ok = 0
    CNT = [0]*(N+1)
    for a,b in AB[:m]:
        L[a].append(b)
        CNT[b]+=1
    d = collections.deque()
    for i in range(1,N+1):
        if CNT[i]==0:
            d.append(i)
    while len(d):
        ok+=1
        i = d.pop()
        for l in L[i]:
            CNT[l]-=1
            if CNT[l]==0:
                d.append(l)
    return ok==N


def meguru_bisect(ng, ok):
    while (abs(ok - ng) > 1):
        mid = (ok + ng) // 2
        if is_ok(mid):
            ok = mid
        else:
            ng = mid
    return ok

N,Q = map(int, input().split())
AB = [list(map(int, input().split())) for _ in range(Q)]


if is_ok(Q):
    print(-1)
else:
    print(meguru_bisect(Q,0)+1)