n = int(input())
if n%2 == 0:
    print(n**2 // 4)
    for i in range(1,n//2+1):
        for j in range(n-2*i+1):
            print(i,i+1+j)
else:
    print(((n+1)*(n//2))//2)
    for i in range(1,n//2+1):
        for j in range(n-2*i+1):
            print(i,i+1+j)