import sys
from collections import deque

def main():
    N, Q = map(int, sys.stdin.readline().split())
    edges = []
    for _ in range(Q):
        A, B = map(int, sys.stdin.readline().split())
        edges.append((A, B))
    
    left = 1
    right = Q
    answer = -1

    while left <= right:
        mid = (left + right) // 2
        adj = [[] for _ in range(N + 1)]
        in_degree = [0] * (N + 1)
        visited = [False] * (N + 1)

        for j in range(mid):
            A, B = edges[j]
            adj[A].append(B)
            in_degree[B] += 1
            visited[A] = True
            visited[B] = True
        
        K = sum(visited)
        q = deque()
        for node in range(1, N + 1):
            if visited[node] and in_degree[node] == 0:
                q.append(node)
        
        processed = 0
        while q:
            u = q.popleft()
            processed += 1
            for v in adj[u]:
                in_degree[v] -= 1
                if in_degree[v] == 0:
                    q.append(v)
        
        if processed < K:
            answer = mid
            right = mid - 1
        else:
            left = mid + 1
    
    print(answer if answer != -1 else -1)

if __name__ == "__main__":
    main()