N=int(input())
X=[[1]*N for i in range(N)]
for i in range(N):
  X[i][i]=0
M=N*(N-1)
for i in range(N):
  for j in range(1,2+i):
    M-=X[i][j%N]
    X[i][j%N]=0
print(M)
for i in range(N):
  for j in range(N):
    if X[i][j]:
      print(i+1,j+1)