import sys
sys.setrecursionlimit(10 ** 9)
import pypyjit
pypyjit.set_param('max_unroll_recursion=-1')

n, q = map(int, input().split())
edges = [[] for _ in range(n)]
for i in range(1, q + 1):
    a, b = map(int, input().split())
    edges[a - 1].append((b - 1, i))

used = [False] * n
used2 = [False] * n

def dfs(pos, x):
    used2[pos] = True
    for npos, i in edges[pos]:
        if i > x:
            continue
        if used2[npos]:
            return True
        if used[npos]:
            continue
        used[npos] = True
        if dfs(npos, x):
            return True
    used2[pos] = False
    return False

def ok(x):
    global used, used2
    used = [False] * n
    used2 = [False] * n
    for i in range(n):
        if used[i]:
            continue
        used[i] = True
        if dfs(i, x):
            return True
    return False

l = 0
r = q + 1

while r - l > 1:
    mid = (l + r) // 2
    if ok(mid):
        r = mid
    else:
        l = mid
if r == q + 1:
    print(-1)
else:
    print(r)