import sys
input = sys.stdin.readline

from collections import defaultdict 

def main():
    n,k = map(int,input().split()); MOD = pow(10,9) + 7
    if k == 0:
        print(1);exit()
    #A = [[0]*n for _ in range(n)]
    G = [[] for _ in range(n)]
    deg = [0]*n
    dic = {}
    for _ in range(k):
        r,c = map(int,input().split())
        r -= 1; c -= 1
        if r == c: #対角成分に1があったら無理
            print(-1);exit()
        G[r].append(c)
        deg[c] += 1
    
    TP = topological(G,deg)
    #print(TP)
    if len(TP) < n:
        print(-1);exit()
    
    dp = [0]*n
    for v in deg:
        dp[v] = 1
    for v in TP:
        for u in G[v]:
            dp[u] = max(dp[u], dp[v] + 1)
    #print(dp)
    ans = max(dp)
    print(ans)

    
    

# degは入次数を記録したもの
def topological(graph, deg):
    start = []
    n = len(deg)
    for i in range(n):
        if deg[i] == 0: #入次数がないものがスタート
            start.append(i)
    topo = []
    while start:
        v = start.pop()
        topo.append(v)
        for u in graph[v]:
            deg[u] -= 1
            if deg[u] == 0:
                start.append(u)
    #トポロジカルソートできない場合は配列がnよりも短くなる。
    return topo


if __name__ == "__main__":
    main()