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


def check(n):
    def dfs(x):
        nv[x] = 0
        for y in E[x]:
            if st[y] == 1:
                return True
            st[y] = 1
            if nv[y] and dfs(y):
                return True
            st[y] = 0

    E = [[] for _ in range(N)]
    for i in range(n):
        a,b = AB[i]
        a -= 1
        b -= 1
        E[a].append(b)
    nv = [1] * N
    for i in range(N):
        if nv[i]:
            st = [0] * N
            st[i] = 1
            if dfs(i):
                return True
    return False


nv = [0] * N
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)