#include <iostream>
using namespace std;
int main() {
    
    int n;
    cin >> n;
    int sum = 0;
    int a[n+1];
    // write your code here......
    for(int i=1;i<=n;i++){
        a[i]=i;
    }
    for(int i=1;i<=n;i++){
        if(a[i]%2==0){
            sum=sum+a[i];
        }
    }

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