#include<stdio.h>
int main(){
  int number;
  //printf("Enter how much number do you want to sort?\n");
  scanf("%d",&number);
  int i, store;
  long long answer=0;
  //printf("Enter %d number", number);
  for (i=1;i<=number;i++){
    scanf("%d", &store);
  }
  for (i=1;i<=store;i++){
    if (i%2==0){
      answer+=i;
    }
  }
  printf("%lld\n", answer);



  return 0;
}
