抄吧，一抄一个不吱声

#include<bits/stdc++.h> 
using namespace std;
int main() {     
    long long n;    
    cin>>n;     
    long long total = n\*(n+1)/2;    
    long long odd = (n+1)/2;    
    long long bad = odd\*(odd+1)/2;    
    cout<<total - bad;           
    return 0; 
}