codeforces 1113A_KLChen的博客

V + 等差求2-(N-1-V)的和

#include<bits/stdc++.h>
using namespace std;
#define fst first
#define sec second
#define sci(num) scanf("%d",&num)
#define scl(num) scanf("%lld",&num)
#define mem(a,b) memset(a,b,sizeof a)
#define cpy(a,b) memcopy(a,b,sizeof b)
typedef long long LL;
typedef pair<int,int> P;
const int MAX_N = 1e5 + 100;
int nums[MAX_N];
int d[MAX_N];
int main() {
    ios::sync_with_stdio(false); cin.tie(0);
    LL N,V;
    cin >> N >> V;
    LL ans = 0;
    LL need = N - 1;
    if (need <= V) ans = need;
    else ans = V + (3 + need - V) * (need - V) / 2;
    cout << ans << endl;



    return 0;
}
#include<bits/stdc+