Problem: We have an N×N checkerboard.

From the square at the upper left corner, a square that is i squares to the right and j squares below is denoted as (i, j). Particularly, the square at the upper left corner is denoted as (0, 0).

Each square (i, j) such that i+j is even, is colored black, and the other squares are colored white.

We will satisfy the following condition by painting some of the white squares:

* Any black square can be reached from square (0, 0) by repeatedly moving to a black square that shares a side with the current square.



Achieve the objective by painting at most 170000 squares black.

Constraints

* 1 \leq N \leq 1,000

Input

The input is given from Standard Input in the following format:


N


Output

Print the squares to paint in the following format:


K
x_1 y_1
x_2 y_2
:
x_K y_K


This means that a total of K squares are painted black, the i-th of which is (x_i, y_i).

Judging

The output is considered correct only if all of the following conditions are satisfied:

* 0 \leq K \leq 170000
* 0 \leq x_i, y_i \leq N-1
* For each i, x_i + y_i is odd.
* If i \neq j, then (x_i, y_i) \neq (x_j, y_j).
* The condition in the statement is satisfied by painting all specified squares.

Examples

Input

2


Output

1
1 0


Input

4


Output

3
0 1
2 1
2 3

Solution (in CPP):

Solution:
// ※※※ 解答不能 ※※※
// tourist氏.
// https://atcoder.jp/contests/cf16-relay-open/submissions/8978522
#include <bits/stdc++.h>
using namespace std;
#define repx(i, a, b) for(int i = a; i < b; i++)
#define rep(i, n) repx(i, 0, n)
#define repr(i, a, b) for(int i = a; i >= b; i--)
#define a first
#define b second

int main(){
    int n;
    scanf("%d", &n);
    vector<pair<int, int>> ret;
    rep(i, n){
        rep(j, n){
            if((i + j) % 2 == 1 && (i % 3 == 0 || i == n - 1 || j == 0 || j == n - 1)) ret.emplace_back(i, j);
        }
    }
    printf("%d\n", ret.size());
    for(auto& p : ret) printf("%d %d\n", p.a, p.b);
    return 0;
}