import sys
from collections import deque

def main():
    n, k = map(int, sys.stdin.readline().split())
    edges = [[] for _ in range(n + 1)]  # 1-based indexing
    in_degree = [0] * (n + 1)
    dp = [0] * (n + 1)

    for _ in range(k):
        r, c = map(int, sys.stdin.readline().split())
        edges[r].append(c)
        in_degree[c] += 1

    q = deque()
    for i in range(1, n + 1):
        if in_degree[i] == 0:
            q.append(i)

    count = 0
    max_length = 0
    while q:
        u = q.popleft()
        count += 1
        for v in edges[u]:
            if dp[v] < dp[u] + 1:
                dp[v] = dp[u] + 1
                if dp[v] > max_length:
                    max_length = dp[v]
            in_degree[v] -= 1
            if in_degree[v] == 0:
                q.append(v)

    if count != n:
        print(-1)
    else:
        print(max_length + 1 if max_length + 1 != 0 else 1)

if __name__ == "__main__":
    main()