N = int(input())
point = []
for pi in range(1, N + 1):
    x, y = map(int, input().split())
    point.append((x, y, pi))
point.sort()

print(len(point) // 2)
for i in range(0, N, 2):
    try:
        print(point[i][2], point[i + 1][2])
    except IndexError:
        exit()