codeforces645B

Mischievous Mess Makers

CodeForces - 645B

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 through n, 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 p_i 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 p_i > p_j.

Input

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

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

Examples

Input

5 2

Output

10

Input

1 10

Output

0

Note

In the first sample, the Mischievous Mess Makers can swap the cows in the stalls 1and 5 during the first minute, then the cows in stalls 2 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.

sol：如果交换次数>(n/2)，答案就是n*（n-1）/2,否则暴力交换，求逆序对数量

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
inline ll read()
{
    ll s=;
    bool f=;
    char ch=' ';
    while(!isdigit(ch))
    {
        f|=(ch=='-'); ch=getchar();
    }
    while(isdigit(ch))
    {
        s=(s<<)+(s<<)+(ch^); ch=getchar();
    }
    return (f)?(-s):(s);
}
#define R(x) x=read()
inline void write(ll x)
{
    if(x<)
    {
        putchar('-'); x=-x;
    }
    if(x<)
    {
        putchar(x+'');    return;
    }
    write(x/);
    putchar((x%)+'');
    return;
}
#define W(x) write(x),putchar(' ')
#define Wl(x) write(x),putchar('\n')
const int N=;
ll n,m,a[N];
struct BIT
{
    ll S[N];
    #define lowbit(x) ((x)&(-x))
    inline void Ins(int x)
    {
        for(;x<=n;x+=lowbit(x))
        {
            S[x]++;
        }
    }
    inline ll Que(int x)
    {
        int Sum=;
        for(;x>;x-=lowbit(x))
        {
            Sum+=S[x];
        }
        return Sum;
    }
}T;
int main()
{
    int i;
    R(n); R(m);
    if(m>=n/) return Wl(n*(n-)>>),;
    else
    {
        for(i=;i<=n;i++) a[i]=i;
        for(i=;i<=m;i++) swap(a[i],a[n-i+]);
//        for(i=1;i<=n;i++) Wl(a[i]);
//        puts("");
        ll ans=;
        for(i=n;i>=;i--)
        {
            ans+=T.Que(a[i]-);
            T.Ins(a[i]);
//            printf("ans=%d\n",ans);
        }
        Wl(ans);
    }
    return ;
}
/*
Input
5 2
Output
10

Input
1 10
Output
0

input
100000 40000
output
4799960000

input
100000 50000
output
4999950000
*/