N = int(input())
D = [0]*(N+1)
E = [(1,2)]
D[1] += 1
D[2] += 1
x = 1
for n in range(3,N+1):
    flag = 0
    for i in range(1,n):
        if D[i]>x:
            D[i] += 1
            D[n] += 1
            E.append((n,i))
        elif D[i]==x and flag==0:
            D[i] += 1
            D[n] += 1
            E.append((n,i))
            flag = 1
    x = D[n]
print(len(E))
for u,v in E:
    print(u,v)