N = int(input())
XY = [list(map(int, input().split())) + [i + 1] for i in range(N)]
XY.sort(key=lambda x: x[1])
XY.sort()
ans = []
for i in range(N // 2):
    ans.append((XY[2 * i][2], XY[2 * i + 1][2]))
print(N // 2)
for x, y in ans:
    print(x, y)