N = int(input())
xy = [tuple(map(int, input().split())) for i in range(N)]
xyi = [(xy[i][0], xy[i][1], i+1) for i in range(N)]
xyi.sort()
print(N//2)
for i in range(N//2):
    print(xyi[2*i][2], xyi[2*i+1][2])