import sys
from collections import deque

def check_cycle(n, edges_list, mid):
    adj = [[] for _ in range(n + 1)]
    in_degree = [0] * (n + 1)
    for i in range(mid):
        a, b = edges_list[i]
        adj[a].append(b)
        in_degree[b] += 1

    q = deque()
    processed = 0
    for node in range(1, n + 1):
        if in_degree[node] == 0:
            q.append(node)

    while q:
        u = q.popleft()
        processed += 1
        for v in adj[u]:
            in_degree[v] -= 1
            if in_degree[v] == 0:
                q.append(v)

    return processed != n

def main():
    input = sys.stdin.read().split()
    ptr = 0
    N = int(input[ptr])
    ptr += 1
    Q = int(input[ptr])
    ptr += 1
    edges = []
    for _ in range(Q):
        a = int(input[ptr])
        ptr += 1
        b = int(input[ptr])
        ptr += 1
        edges.append((a, b))

    low = 1
    high = Q
    answer = -1

    while low <= high:
        mid = (low + high) // 2
        has_cycle = check_cycle(N, edges, mid)
        if has_cycle:
            if mid == 1:
                answer = mid
                break
            else:
                prev_has_cycle = check_cycle(N, edges, mid - 1)
                if not prev_has_cycle:
                    answer = mid
                    break
                else:
                    high = mid - 1
        else:
            low = mid + 1

    print(answer if answer != -1 else -1)

if __name__ == '__main__':
    main()