#include<bits/stdc++.h>
using namespace std;
int main(){
    int a=1,c=1,b;
    cin>>b;
    while(a<=b){
		c=a+c;
		a=a+2;
    }
    cout<<c-1;
}
/**************************************************************
	Problem: 1003
	User: toughuatao1
	Language: C++
	Result: Accepted
	Time:15 ms
	Memory:2072 kb
****************************************************************/