import sys
from collections import deque

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
    ans = -1
    
    while low <= high:
        mid = (low + high) // 2
        adj = [[] for _ in range(N + 1)]
        in_degree = [0] * (N + 1)
        for i in range(mid):
            a, b = edges[i]
            adj[a].append(b)
            in_degree[b] += 1
        
        q = deque()
        for node in range(1, N + 1):
            if in_degree[node] == 0:
                q.append(node)
        
        count = 0
        while q:
            u = q.popleft()
            count += 1
            for v in adj[u]:
                in_degree[v] -= 1
                if in_degree[v] == 0:
                    q.append(v)
        
        if count == N:
            low = mid + 1
        else:
            ans = mid
            high = mid - 1
    
    print(ans if ans != -1 else -1)

if __name__ == "__main__":
    main()