CodeForces 645B Mischievous Mess Makers

简单题。

第一次交换$1$和$n$，第二次交换$2$和$n-1$，第三次交换$3$和$n-2$.....计算一下就可以了。

#pragma comment(linker, "/STACK:1024000000,1024000000")
#include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
#include<vector>
#include<map>
#include<set>
#include<queue>
#include<stack>
#include<iostream>
using namespace std;
typedef long long LL;
const double pi=acos(-1.0),eps=1e-6;
void File()
{
    freopen("D:\\in.txt","r",stdin);
    freopen("D:\\out.txt","w",stdout);
}
template <class T>
inline void read(T &x)
{
    char c=getchar(); x=0;
    while(!isdigit(c)) c=getchar();
    while(isdigit(c)) {x=x*10+c-'0'; c=getchar();}
}

int n,k;

int main()
{
    scanf("%d%d",&n,&k);
    if(k>n/2) k=n/2;
    LL ans=0; int num=n;
    for(int i=1;i<=k;i++)
    {
        ans=ans+num-1+num-2;
        num=num-2;
    }
    printf("%lld\n",ans);
    return 0;
}