N = int(input())
lst = []
for i in range(N):
    x, y = map(int, input().split())
    lst.append([x, y, i])
lst.sort()

print(N // 2)
for i in range(N // 2):
    print(lst[i * 2][2] + 1, lst[i * 2 + 1][2] + 1)