N=int(input())

E=[]
for u in range(1,N//2+1):
    for v in range(u+1,N-u+2):
        E.append((u,v))

"""
D=[0]*(N+1)
for u,v in E:
    D[u]+=1
    D[v]+=1
"""

print(len(E))
for u,v in E:
    print(u,v)