mod = 1000000007
eps = 10**-9


def main():
    import sys
    from collections import deque
    input = sys.stdin.buffer.readline

    N, Q = map(int, input().split())
    AB = [(0, 0)]
    for _ in range(Q):
        AB.append(tuple(map(int, input().split())))

    ok = Q + 1
    ng = 0
    mid = (ok + ng) // 2
    while ok - ng > 1:
        adj = [[] for _ in range(N + 1)]
        in_num = [0] * (N + 1)
        for a, b in AB[1:mid+1]:
            adj[a].append(b)
            in_num[b] += 1
        que = deque()
        seen = [0] * (N + 1)
        for v in range(1, N + 1):
            if in_num[v] == 0:
                seen[v] = 1
                que.append(v)
        while que:
            v = que.popleft()
            for u in adj[v]:
                in_num[u] -= 1
                if in_num[u] == 0:
                    if not seen[u]:
                        seen[u] = 1
                        que.append(u)
        if sum(seen) == N:
            ng = mid
        else:
            ok = mid
        mid = (ok + ng) // 2
    if ok == Q + 1:
        print(-1)
    else:
        print(ok)


if __name__ == '__main__':
    main()