import sys
sys.setrecursionlimit(10 ** 9)

n, k = map(int, input().split())
edges = [[] for _ in range(n)]
for _ in range(k):
    r, c = map(int, input().split())
    r -= 1
    c -= 1
    edges[r].append(c)
    
dist = [-1] * n
used = [False] * n

def dfs(pos):
    dist[pos] = max(dist[pos], 0)
    for npos in edges[pos]:
        if used[npos]:
            print(-1)
            exit()
        if dist[npos] != -1:
            dist[pos] = max(dist[pos], dist[npos] + 1)
            continue
        dfs(npos)
        dist[pos] = max(dist[pos], dist[npos] + 1)

for i in range(n):
    if dist[i] != -1:
        continue
    used[i] = True
    dfs(i)
    used[i] = False
    
print(max(dist) + 1)