645B - Mischievous Mess Makers - CodeForces Solution

#include <bits/stdc++.h>

using namespace std;

#define fi first

#define se second

#define int long long

#define pb push_back

#define MOD 1000000007

int bigmod (int b, int p){

	if (p==0) return 1LL;

	if (p==1) return b%MOD;

	int cur=bigmod(b, p/2);

	cur=(cur*cur)%MOD;

	if (p%2==0) return cur;

	else return (b*cur)%MOD;

}

void solve(){

    int n, k;

    cin>>n>>k;

    n--;

    int cur=n;

    int ans=0;

    for (int i=1; i<=k; i++){

        if (cur<1) break;

        ans+=cur+cur-1;

        cur-=2;

    }

    cout<<ans<<endl;

}

signed main(){

    int t;

    t=1;

    while (t--) solve();

}