1178_D. Prime Graph_32146
Every person likes prime numbers. Alice is a person, thus she also shares the love for them. Bob wanted to give her an affectionate gift but couldn't think of anything inventive. Hence, he will be giving her a graph. How original, Bob! Alice will surely be thrilled!

When building the graph, he needs four conditions to be satisfied: 

  * It must be a simple undirected graph, i.e. without multiple (parallel) edges and self-loops. 
  * The number of vertices must be exactly n — a number he selected. This number is not necessarily prime. 
  * The total number of edges must be prime. 
  * The degree (i.e. the number of edges connected to the vertex) of each vertex must be prime. 



Below is an example for n = 4. The first graph (left one) is invalid as the degree of vertex 2 (and 4) equals to 1, which is not prime. The second graph (middle one) is invalid as the total number of edges is 4, which is not a prime number. The third graph (right one) is a valid answer for n = 4. 

<image>

Note that the graph can be disconnected.

Please help Bob to find any such graph!

Input

The input consists of a single integer n (3 ≤ n ≤ 1 000) — the number of vertices.

Output

If there is no graph satisfying the conditions, print a single line containing the integer -1.

Otherwise, first print a line containing a prime number m (2 ≤ m ≤ (n(n-1))/(2)) — the number of edges in the graph. Then, print m lines, the i-th of which containing two integers u_i, v_i (1 ≤ u_i, v_i ≤ n) — meaning that there is an edge between vertices u_i and v_i. The degree of each vertex must be prime. There must be no multiple (parallel) edges or self-loops.

If there are multiple solutions, you may print any of them.

Note that the graph can be disconnected.

Examples

Input


4


Output


5
1 2
1 3
2 3
2 4
3 4

Input


8


Output


13
1 2
1 3
2 3
1 4
2 4
1 5
2 5
1 6
2 6
1 7
1 8
5 8
7 8

Note

The first example was described in the statement.

In the second example, the degrees of vertices are [7, 5, 2, 2, 3, 2, 2, 3]. Each of these numbers is prime. Additionally, the number of edges, 13, is also a prime number, hence both conditions are satisfied.

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def SieveOfEratosthenes(n):
	prime = [True for i in range(n + 1)]
	p = 2
	while (p * p <= n):
		if (prime[p] == True):

			for i in range(p * p, n + 1, p):
				prime[i] = False
		p += 1
	pp=[]
	for p in range(2, n + 1):
		if prime[p]:
			pp.append(p)
	return pp
def f(n):
	Primes=SieveOfEratosthenes(10000)
	deg=0
	for i in Primes:
		if i>=n:
			deg=i
			break
	a=[]
	cnt=[0]*(n)
	cnt[0]+=1
	cnt[n-1]+=1
	a.append((1,n))
	for i in range(n-1):
		a.append((i+1,i+2))
		cnt[i]+=1
		cnt[i+1]+=1
	req=deg-len(a)
	l=0
	r=n-2
	c=2
	for i in range(req):
		a.append((l+1,r+1))
		cnt[l]+=1
		cnt[r]+=1
		if r==l+1:
			l=0
			c+=1
			r=n-c
		else:
			l+=1
			r-=1
	print(len(a))
	for i in a:
		print(*i)
	return ''
print(f(int(input())))