m=1000
t=50
id=-1
def g():
 global id
 id+=1
 return id
e=[]
rs=[]
for _ in range(m):
 i=g()
 for _ in range(t):
  j=g()
  e+=[(i,j)]
 rs+=[i]
while len(rs)>1:
 nrs=[]
 for l in range(0,len(rs),t-2):
  r=min(len(rs),l+t-2)
  p=g()
  for i in rs[l:r]:
   e+=[(p,i)]
  nrs+=[p]
 rs=nrs
n=g()
print(n)
for u,v in e:
 print(u+1,v+1)