Score : 400 points
For an integer N, we will choose a permutation \{P_1, P_2, ..., P_N\} of \{1, 2, ..., N\}.
Then, for each i=1,2,...,N, let M_i be the remainder when i is divided by P_i.
Find the maximum possible value of M_1 + M_2 + \cdots + M_N.
Input is given from Standard Input in the following format:
N
Print the maximum possible value of M_1 + M_2 + \cdots + M_N.
2
1
When the permutation \{P_1, P_2\} = \{2, 1\} is chosen, M_1 + M_2 = 1 + 0 = 1.
13
78
1
0