I construct vertical blocks made of bricks. Now, blocks are of height 1,2,3,...n-1. Now, to make these many i need n*(n-1)/2 bricks. But, I have n bricks so I have to remove former minus latter number of bricks, which is n(n-3)/2. Now, the problem can be changed to number of ways of obtaining n(n-3)/2 from numbers 1,2,3,...n-1 while each number can be used only once. If you find the approach to be correct, please tell me how to obtain this number.(I am a newbie in coding..:)).