Math khong kho

Code

#include <bits/stdc++.h>
using namespace std;

int main(){
	long long n, m;
	cin >> n >> m;
	long long sum[3];
	fill(sum +1, sum +3, 0);
	//sum[1] = sum(1,2,3,...) 
	//sum[2] = sum(n,n-1,n-2,..)
	long long l;
	long long r;
	if (m %2 == 1){
		l = 1; 
		r = m/2 + 1;
		sum[1] = (r + l) * r /2;
		l = n - (m/2) + 1;
		r = n;
		sum[2] = (r + l) * (r-l+1) /2;
	} else {
		l = 1; 
		r = m/2;
		sum[1] = (r + l) * r /2;
		l = n - (m/2) + 1;
		r = n;
		sum[2] = (r + l) * (r-l+1) /2;
	}
	cout << sum[1] + sum[2];
	return 0;
}