import sys
from sys import stdin
sys.setrecursionlimit(1 << 25)

def main():
    N, Q = map(int, stdin.readline().split())
    adj = [[] for _ in range(N+1)]  # Original graph adjacency list
    for i in range(Q):
        A, B = map(int, stdin.readline().split())
        # Check if B can reach A in the current graph (before adding this edge)
        visited = [False] * (N + 1)
        stack = [B]
        visited[B] = True
        found = False
        while stack:
            u = stack.pop()
            if u == A:
                found = True
                break
            for v in adj[u]:
                if not visited[v]:
                    visited[v] = True
                    stack.append(v)
        if found:
            print(i+1)
            return
        adj[A].append(B)
    print(-1)

if __name__ == "__main__":
    main()