import sys
input = sys.stdin.readline

from collections import defaultdict 

def main():
    n,k = map(int,input().split())
    if k == 0:
        print(1);exit()
    #A = [[0]*n for _ in range(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()
        if r not in dic:
            dic[r] = [c]
        else:
            dic[r].append(c)
    #print(dic)
    for i in range(1,2*pow(10,5)+100):
        if not dic: print(i);exit()
        predic = {}
        dic,predic = predic,dic
        for r in predic:
            for nr in predic[r]:
                if nr not in predic: continue
                nc = predic[nr]
                if r in nc: print(-1);exit()
                dic[r] = nc
        #print(dic)

if __name__ == "__main__":
    main()