import sys
from collections import deque

def main():
    input = sys.stdin.read().split()
    idx = 0
    N = int(input[idx])
    idx += 1
    Q = int(input[idx])
    idx += 1
    edges = []
    for _ in range(Q):
        a = int(input[idx])
        idx += 1
        b = int(input[idx])
        idx += 1
        edges.append((a, b))
    
    left = 1
    right = Q
    ans = -1
    
    while left <= right:
        mid = (left + right) // 2
        adj = [[] for _ in range(N + 1)]
        in_degree = [0] * (N + 1)
        for a, b in edges[:mid]:
            adj[a].append(b)
            in_degree[b] += 1
        
        queue = deque()
        for i in range(1, N + 1):
            if in_degree[i] == 0:
                queue.append(i)
        
        cnt = 0
        while queue:
            u = queue.popleft()
            cnt += 1
            for v in adj[u]:
                in_degree[v] -= 1
                if in_degree[v] == 0:
                    queue.append(v)
        
        if cnt != N:
            ans = mid
            right = mid - 1
        else:
            left = mid + 1
    
    print(ans if ans != -1 else -1)

if __name__ == "__main__":
    main()