import sys
sys.setrecursionlimit(100050)
N,Q = map(int,input().split())
AB = [list(map(int,input().split())) for _ in range(Q)]

def check(n):
    E = [[] for _ in range(N)]
    D = [0] * N
    for i in range(n):
        a,b = AB[i]
        a -= 1
        b -= 1
        E[a].append(b)
        D[b] += 1
    Q = []
    for i in range(N):
        if D[i] == 0:
            Q.append(i)
    while Q:
        x = Q.pop()
        for y in E[x]:
            D[y] -= 1
            if D[y] == 0:
                Q.append(y)
    return sum(D) > 0

nv = []
lb = 0
ub = Q + 1
while ub - lb > 1:
    mid = (ub + lb) // 2
    if check(mid):
        ub = mid
    else:
        lb = mid


if ub == Q + 1:
    print(-1)
else:
    print(ub)