https://www.acmicpc.net/problem/18127
#pragma GCC target("avx,avx2,fma")
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h>
#define fastio ios::sync_with_stdio(0), cin.tie(0), cout.tie(0)
#define int int64_t
using namespace std;
int32_t main(){
fastio;
int a,b,ret = 0,temp = 1; cin >> a >> b;
for(int i = 0; i <= b; i++){
ret += temp;
temp += a - 2;
}
cout << ret << "\n";
}
각 단계에 따라 A - 2만큼 증가합니다