N = int(input())
XY = [list(map(int, input().split())) for _ in range(N)]

XY = sorted(enumerate(XY), key=lambda x:x[1])
ans = []
for i in range(N//2):
    ans.append((XY[i*2][0]+1, XY[i*2+1][0]+1))

print(len(ans))
for x, y in ans:
    print(x, y)