#include <stdio.h>

int main(){
    int i,n,s = 0;
    scanf("%d",&n);
    for(i = 1;i <= n;i = i + 2){
       s = s + i;
    }
    printf("%d\n",s);
    
    return 0;
}
/**************************************************************
	Problem: 1003
	User: admin
	Language: C
	Result: Accepted
	Time:10 ms
	Memory:1144 kb
****************************************************************/