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
    
    adj = [[] for _ in range(N+1)]  # 1-based
    
    for step in range(1, Q+1):
        A = int(input[idx]); idx +=1
        B = int(input[idx]); idx +=1
        
        # Check if B can reach A
        visited = [False] * (N +1)
        dq = deque()
        dq.append(B)
        visited[B] = True
        found = False
        while dq:
            u = dq.popleft()
            if u == A:
                found = True
                break
            for v in adj[u]:
                if not visited[v]:
                    visited[v] = True
                    dq.append(v)
        if found:
            print(step)
            return
        
        # Add the edge
        adj[A].append(B)
    
    print(-1)

if __name__ == "__main__":
    main()