题解 | 求 1 - n 之间偶数的和

#include <iostream>
using namespace std;

int main() {
    
    int n;
    cin >> n;
    int sum = 0;
    
    // write your code here......
    int temp = n * (n + 1) / 2;
    sum = (n % 2 == 0 ? (temp + n/2) / 2 : (temp + n/2) / 2);

    cout << sum << endl;
    
    return 0;
}