from sys import stdin

n, q, *indata = map(int, stdin.read().split())
offset = 0
left = 0
right = q+1
while right - left >= 2:
    mid = (right + left) // 2
    g = [[] for i in range(n)]
    for i in range(mid):
        s, t = indata[i*2]-1,indata[i*2+1]-1
        g[s].append(t)
    check = [0 for j in range(n)]
    cyclecheck = False
    for j in range(n):
        if check[j] == 0:
            que = [(j,0)]
            while que:
                now, inout = que.pop()
                if inout == 0:
                    if check[now] == 0:
                        que.append((now,1))
                        check[now] = 1
                        for k in g[now]:
                            if check[k] == 1:
                                cyclecheck = True
                                break
                            elif check[k] == 0:
                                que.append((k,0))
                        if cyclecheck:
                            break
                else:
                    check[now] = 2
            if cyclecheck:
                break
    if cyclecheck:
        right = mid
    else:
        left = mid

if right == mid:
    print("{}".format(-1))
else:
    print("{}".format(right))