import sys

# sys.setrecursionlimit(200005)
int1 = lambda x: int(x)-1
pDB = lambda *x: print(*x, end="\n", file=sys.stderr)
p2D = lambda x: print(*x, sep="\n", end="\n\n", file=sys.stderr)
def II(): return int(sys.stdin.readline())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def LI1(): return list(map(int1, sys.stdin.readline().split()))
def LLI1(rows_number): return [LI1() for _ in range(rows_number)]
def SI(): return sys.stdin.readline().rstrip()
dij = [(0, 1), (-1, 0), (0, -1), (1, 0)]
# dij = [(0, 1), (-1, 0), (0, -1), (1, 0), (1, 1), (1, -1), (-1, 1), (-1, -1)]
inf = (1 << 63)-1
# inf = (1 << 31)-1
md = 10**9+7
# md = 998244353

def topo(st):
    global cnt
    while st:
        u = st.pop()
        for v, i in to[u]:
            if fin[i]: continue
            deg[v] -= 1
            fin[i] = 1
            cnt += 1
            if deg[v] == 0:
                st.append(v)

n, q = LI()

to = [[] for _ in range(n)]
ee = []
deg = [0]*n

for i in range(q):
    u, v = LI1()
    to[u].append((v, i))
    deg[v] += 1
    ee.append((u, v))

cnt = 0
st = [u for u in range(n) if deg[u] == 0]
fin = [0]*q
topo(st)

if cnt == q:
    print(-1)
    exit()

for i in range(q)[::-1]:
    if fin[i]: continue
    fin[i] = 1
    u, v = ee[i]
    deg[v] -= 1
    cnt += 1
    if deg[v] == 0: topo([v])
    if cnt == q:
        print(i+1)
        break
    # print(cnt, fin)