import sys

int1 = lambda x: int(x) - 1

# input = lambda: sys.stdin.buffer.readline()
input = lambda: sys.stdin.readline().rstrip()
ii = lambda: int(input())
i1 = lambda: int1(input())
mi = lambda: map(int, input().split())
mi1 = lambda: map(int1, input().split())
li = lambda: list(mi())
li1 = lambda: list(mi1())
lli = lambda n: [li() for _ in range(n)]

INF = float("inf")
mod = int(1e9 + 7)
# mod = 998244353

n, q = mi()
g = [list() for i in range(n)]
for i in range(q):
    a, b = mi1()
    g[a].append((b, i))

ok = -1
ng = q
while ng - ok > 1:
    mid = ok + ng >> 1
    flag = False

    # check
    f = [-1] * n
    idx = [0] * n
    st = []
    for i in range(n):
        if f[i] != -1:
            continue
        st.append(i)
        while st:
            cur = st.pop()
            if idx[cur] == 0:
                f[cur] = i
            while idx[cur] < len(g[cur]) and g[cur][idx[cur]][1] > mid:
                idx[cur] += 1
            if idx[cur] == len(g[cur]):
                f[cur] = -2
            else:
                to = g[cur][idx[cur]][0]
                if f[to] == i:
                    flag = True
                    st = []
                    break
                elif f[to] == -1:
                    st.append(cur)
                    st.append(to)
                    idx[cur] += 1
                elif f[to] == -2:
                    st.append(cur)
                    idx[cur] += 1
        if flag:
            break
    #

    if flag:
        ng = mid
    else:
        ok = mid
if ng == q:
    print(-1)
else:
    print(ng + 1)