It is a balmy spring afternoon, and Farmer John's n cows are ruminating about link-cut cacti in their stalls. The cows, labeled 1 throughn, are arranged so that the i-th cow occupies the i-th stall from the left. However, Elsie, after realizing that she will forever live in the shadows beyond Bessie's limelight, has formed the Mischievous Mess Makers and is plotting to disrupt this beautiful pastoral rhythm. While Farmer John takes his k minute long nap, Elsie and the Mess Makers plan to repeatedly choose two distinct stalls and swap the cows occupying those stalls, making no more than one swap each minute.

Being the meticulous pranksters that they are, the Mischievous Mess Makers would like to know the maximum messiness attainable in the k minutes that they have. We denote as pi the label of the cow in the i-th stall. The messiness of an arrangement of cows is defined as the number of pairs (i, j) such that i < j and pi > pj.

The first line of the input contains two integers n and k (1 ≤ n, k ≤ 100 000) — the number of cows and the length of Farmer John's nap, respectively.

Output a single integer, the maximum messiness that the Mischievous Mess Makers can achieve by performing no more than k swaps.

In the first sample, the Mischievous Mess Makers can swap the cows in the stalls 1 and 5 during the first minute, then the cows in stalls2 and 4 during the second minute. This reverses the arrangement of cows, giving us a total messiness of 10.

In the second sample, there is only one cow, so the maximum possible messiness is 0.

题意：问最多进行k此互换后,the number of pairs (i, j) such that i < j and pi > pj.

思路：由贪心可知,每次互换最外边的一对可以得到数目最大,每换一次最外边的一对,number增加2*n-3;

AC代码：

#include <bits/stdc++.h>

using namespace std;

int main()

{

    long long n,k,ans=;

    cin>>n>>k;

        while(k--&&n>)

        {

            ans+=*n-;

            n-=;

        }

        cout<<ans<<endl;

    return ;

}