import sys
input = sys.stdin.readline
sys.setrecursionlimit(10**5+10)
from collections import *

def dfs(v, pv, t):
    global pos
    
    seen[v] = True
    hist.append(v)
    
    for i, nv in G[v]:
        if t<i:
            continue
        
        if finished[nv]:
            continue
        
        if seen[nv] and not finished[nv]:
            pos = nv
            return
        
        dfs(nv, v, t)
        
        if pos!=-1:
            return
    
    hist.pop()
    finished[v] = True

N, Q = map(int, input().split())
G = [[] for _ in range(N)]

for i in range(Q):
    A, B = map(int, input().split())
    G[A-1].append((i+1, B-1))

l, r = 0, Q

while l<=r:
    m = (l+r)//2
    seen = [False]*N
    finished = [False]*N
    pos = -1
    hist = []
    
    for v in range(N):
        if not seen[v]:
            dfs(v, -1, m)
            
            if len(hist)>0:
                r = m-1
                break
    else:
        l = m+1

ans = l

if ans==Q+1:
    print(-1)
else:
    print(ans)