from sys import stdin
n, m, *indata = map(int, stdin.read().split())
offset = 0
g = [[] for i in range(n+1)]
for i in range(m):
    s, t = indata[offset + 2*i],indata[offset + 2*i+1]
    g[s].append(t)

check = [False for i in range(n+1)]
for i in range(1,n+1):
    if not check[i]:
        que = [(i,0)]
        visited = set()
        while que:
            now, inout = que.pop()
            if inout == 0:
                if not check[now]:
                    check[now] == True
                    que.append((now,1))
                    visited.add(now)
                    for j in g[now]:
                        if j in visited:
                            print(-1)
                            exit()
                        que.append((j,0))
            else:
                visited.remove(now)

dp = [0 for i in range(n+1)]
check = [False for i in range(n+1)]
for i in range(1,n+1):
    if not check[i]:
        que = [(i,0)]
        check[i] = True
        while que:
            now, inout = que.pop()
            if inout == 0:
                que.append((now,1))
                for j in g[now]:
                    if not check[j]:
                        que.append((j,0))
            else:
                for j in g[now]:
                    dp[now] = max(dp[now],dp[j]+1)

ans = max(dp) + 1
print("{}".format(ans))