n, m = map(int, input().split())
g = [[] for _ in range(n)]
g2 = [[] for _ in range(n)]
inv = [0] * n
for i in range(m):
    a, b = map(int, input().split())
    g[a - 1].append(b - 1)
    g2[b - 1].append(a - 1)
    inv[b - 1] += 1
s = []
d = [-1] * n
for i in range(n):
    if inv[i] == 0:
        s.append(i)
        d[i] = 0
while s:
    p = s.pop()
    for node in g[p]:
        inv[node] -= 1
        if inv[node] == 0:
            t = -1
            for node2 in g2[node]:
                if t < d[node2]:
                    t = d[node2]
            d[node] = d[node2] + 1
            s.append(node)
if -1 in d:
    print(-1)
else:
    print(max(d) + 1)