"""

N-1,1,1,1,1,1,1,1,1
N-1,N-2,2,2,2,2,2,2,1
N-1,N-2,N-3,3,3,3,3,3

"""

N = int(input())

lis = [ [0] * N for i in range(N) ]
e = [0] * N

for i in range(N):
    
    for j in range(i+1,N-i):
        e[i] += 1
        e[j] += 1
        lis[i][j] = lis[j][i] = 1

ans = []
for j in range(N):
    for i in range(j):
        if lis[i][j] == 1:
            ans.append((i+1,j+1))

print (len(ans))
for i in ans:
    print (*i)