import sys
input = sys.stdin.readline

N = int(input())
X, Y = [0] * N, [0] * N
M = 8005
D = [[] for i in range(M)]
for i in range(N):
    X[i], Y[i] = map(int, input().split())
    D[Y[i]].append((X[i], i))

ans = []
pre = -1
for i in range(M):
    if not D[i]:
        continue
    D[i].sort()
    if pre != -1:
        _, z = D[i].pop()
        ans.append((pre, z + 1))        
    while len(D[i]) >= 2:
        _, z = D[i].pop()
        _, w = D[i].pop()
        ans.append((z + 1, w + 1))
    if D[i]:
        _, pre = D[i].pop()
        pre += 1
    else:
        pre = -1
        
print(len(ans))
for a, b in ans:
    print(a, b)