from sys import setrecursionlimit


setrecursionlimit(10 ** 6)


def DFS(v):
    seen[v] = 1
    for i in g[v]:
        if seen[i] == 1 or (seen[i] == 0 and DFS(i)):
            return True
    seen[v] = 2
    return False


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

while abs(left - right) > 1:
    mid = (right + left) // 2

    g = [[] for _ in range(n)]
    for a, b in edges[:mid]:
        g[a].append(b)
    seen = [0] * n
    has_cycle = False
    for i in range(n):
        if seen[i] == 0 and DFS(i):
            has_cycle = True

    if has_cycle:
        right = mid
    else:
        left = mid

ans = right
print(ans if ans != q + 1 else -1)