#include<stdio.h> 
int main()
{
    int i,n,s=0;
    scanf("%d",&n); 
    for(i=1; i<=n ;i+=2)
    {
     s=s+i;
    }
    printf("%d",s);
    return 0;
}
/**************************************************************
	Problem: 1110
	User: admin
	Language: C
	Result: Accepted
	Time:8 ms
	Memory:1144 kb
****************************************************************/