n,k = map(int,input().split())
g = [[] for _ in range(n)]
deg = [0]*n
for _ in range(k):
    a,b = map(int,input().split())
    g[a-1].append(b-1)
    deg[b-1] += 1

dp = [0]*n
q = [i for i in range(n) if deg[i]==0]
for i in q: dp[i] = 1
while q:
    v = q.pop()
    for c in g[v]:
        dp[c] = max(dp[c],dp[v]+1)
        deg[c] -= 1
        if deg[c] == 0:
            q.append(c)
    
print(-1 if min(dp) == 0 else max(dp))