mod = 1000000007
eps = 10**-9


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

    def fin():
        print(-1)
        exit()

    N, K = map(int, input().split())
    adj = [[] for _ in range(N+1)]
    adj_rev = [[] for _ in range(N+1)]
    in_num = [0] * (N+1)
    for _ in range(K):
        r, c = map(int, input().split())
        if r == c:
            fin()
        adj[r].append(c)
        adj_rev[c].append(r)
        in_num[c] += 1
    que = deque()
    for v in range(1, N+1):
        if in_num[v] == 0:
            que.append(v)
    seq = []
    while que:
        v = que.popleft()
        seq.append(v)
        for u in adj[v]:
            in_num[u] -= 1
            if in_num[u] == 0:
                que.append(u)
    if len(seq) != N:
        fin()
    dp = [0] * (N+1)
    seq.reverse()
    for v in seq:
        for u in adj[v]:
            dp[v] = max(dp[v], dp[u] + 1)
    print(max(dp) + 1)


if __name__ == '__main__':
    main()