//sum of all odd numbers 
#include<stdio.h>
int main(){
  int sum,a;
  scanf("%d",&a);
  if(a<1){
      printf("invalid input");
  }
  else if{
      for(int i=1;i<=a;i++){
          if(i%2!=0){
              sum=sum+i;
          }
      }
      printf("%d",sum);
  }
}