CF41E.3-cycles

普及/提高-

通过率:0%

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题目描述

During a recent research Berland scientists found out that there were nn cities in Ancient Berland, joined by two-way paths. Any two cities are joined by no more than one path. No path joins a city with itself. According to a well-known tradition, the road network was built so that it would be impossible to choose three cities from each of which one can get to any other one directly. That is, there was no cycle exactly as long as 3. Unfortunately, the road map has not been preserved till nowadays. Now the scientists are interested how much developed a country Ancient Berland was. Help them - find, what maximal number of roads could be in the country. You also have to restore any of the possible road maps.

输入格式

The first line contains an integer nn ( 1<=n<=1001<=n<=100 ) — the number of cities in Berland.

输出格式

On the first line must be printed number mm — the maximal number of roads in Berland. Then print mm lines containing two numbers each — the numbers of cities that the given road joins. The cities are numbered with integers from 11 to nn . If there are several variants of solving the problem, print any of them.

输入输出样例

  • 输入#1

    3
    

    输出#1

    2
    1 2
    2 3
    
  • 输入#2

    4
    

    输出#2

    4
    1 2
    2 3
    3 4
    4 1
    
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