N = int(input())

if N==2:
    exit(print(0))
    
edges = [(2, 3)]
now = 3
need = 1

while N-now>=need:
    for i in range(need):
        edges.append((now, now+i+1))
    
    now += need
    need += 1

print(len(edges))

for u, v in edges:
    print(u, v)